Javascript Type Conversion

6 Key excerpts on "Javascript Type Conversion"

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  eBook - ePub

  Clean Code in JavaScript

  Develop reliable, maintainable, and robust JavaScript

  • James Padolsey(Author)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  In most situations, we will need to detect a plain object explicitly. Instead, we should rely only on the interface or data that it provides us. If a user of our abstraction wishes to pass us a non-plain object but it still has the properties that we require, then who are we to complain?

  Passage contains an image

  Conversion, coercion, and casting

  So far, we have learned how to tell the difference between various types and characteristics within JavaScript using detection. As we have seen, detection is useful when needing to provide alternative values or warnings in the case of unexpected or incompatible values. There is an additional mechanism for dealing with such values, however: we can convert them from the values we don't desire into the values we do desire.

  In order to convert a value, we use a mechanism known as casting . Casting is the intentional and explicit derivation of one type from another type. In contrast to casting, there is also coercion . Coercion is the implicit and internal process of conversion employed by JavaScript when we use operators or language constructs that require specific types. An example of this would be when passing String values to a multiplication operator. The operator will naturally coerce its String operands to numbers so that it can attempt to multiply them:

  '5' * '2'; // => 10 (Number)

  The underlying mechanisms in both casting and coercion are identical. They are both mechanisms of conversion. But how we access these low-level behaviors is key. If we do so explicitly, clearly communicating our intent, then the readers of our code will have a far nicer time.

  Consider the following code, which contains two different mechanisms for converting a String into a Number: Number('123'); // => 123+'123'; // => 123

  Here, we are using two different techniques to force the conversion of a value from a String into a Number. The Number() constructor, when called as a function, will internally convert the passed value into a Number primitive. The unary + operator will do the same, although it is arguably less clear. Coercion is even less clear as it often appears to occur as a side effect of some other operation. Here are some examples of this:

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  eBook - PDF

  JavaScript All-in-One For Dummies

  • Chris Minnick(Author)
  • 2023(Publication Date)
  • For Dummies

    (Publisher)

  68 BOOK 1 JavaScript Fundamentals As you’ll see when you start working with JavaScript objects, arrays, and modules, there are exceptions to every rule. Taking a Look at the Data Types Variables and constants all have two things in common: a value and a type. You’ve already seen examples of values. These are the bits of data that you want to store in the variable. In this section, I talk about type. A variable’s data type is the kind of data a variable can hold. It’s what determines whether 97103-4534 is a postal code or a mathematical operation that results in 92569. JavaScript is loose and dynamic JavaScript is a loosely typed language. What this means is that you can store any type of data in a variable or constant without having to tell JavaScript in advance the type of data you’ll store in the variable. JavaScript is also a dynamically typed language. This means that you can change the type of data stored in a variable. You might initialize a variable using a number but later store text in it. Though changing the type of a variable is unusual and generally should be avoided, JavaScript tries to be friendly and doesn’t complain. To see a value’s or variable’s data type, you can use the typeof operator. To try it out, open your JavaScript console and try entering the following expressions: » typeof "1" » typeof 0 » typeof true » typeof "true" » typeof a » typeof "a" The result of running these expressions is shown in Figure 3-3. Using Data CHAPTER 3 Using Data 69 Passing by value JavaScript has seven basic data types, which are known as the primitive data types. Primitive data types are passed by value. When you create a new variable from an existing variable, what’s happening is that the value of the existing vari- able is copied (as if you were taking a picture of it) to the new one. Let’s look at an example to better understand the implications of this concept.

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  eBook - PDF

  Web Programming

  Building Internet Applications

  • Chris Bates(Author)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  This is good practice and it will give you a real feel for the language. 6.4. Variables 157 Converting data types Always assume that a value in JavaScript is a string unless you have explicitly converted it to a number. The interpreter will try to best-guess your intentions if they are not clear. Here is an example of the confusion which can arise. 1 5 30 158 6. An Introduction to JavaScript Figure 6.4 Converting between data types Two numerical values are stored as strings. The two values are then converted to Integers and stored in two more variables. This gives us two copies of each variable. These are printed individually to the document and display as expected, see Figure 6.4. The initial values are joined together and displayed once more. Because the values are held as strings, JavaScript concatenates one onto the end of the other when it processes the + operator. The Integer values created in the conversion are now added together and the result stored in the variable result. This is displayed and gives the answer we might expect. Finally, the confusing part. The two integers are added together within a call to the document.writeln method. This places the integer values in a context which expects strings as its values.

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  No longer available |Learn more

  The JavaScript Workshop

  A New, Interactive Approach to Learning JavaScript

  • Joseph Labrecque, Jahred Love, Daniel Rosenbaum, Nick Turner, Gaurav Mehla, Alonzo L. Hosford, Florian Sloot, Philip Kirkbride(Authors)
  • 2019(Publication Date)
  • Packt Publishing

    (Publisher)

  5. Beyond the Fundamentals

  Overview

  By the end of this chapter, you will be able to identify the difference between JavaScript's mutable and immutable types; manipulate each of the built-in data types confidently; convert data from one type to another; format data types for presentation; and differentiate between an expression and a statement.

  Introduction

  In the previous chapter, you were given a tour of JavaScript, its runtimes, and its history. Using a high-level topography, that chapter will have given you an idea as to what JavaScript is, what it can do, and its ubiquity within the internet software development industry.

  Understanding code can be difficult for beginners. JavaScript is no exception. Its flexibility, extensive language syntax, and varying coding patterns can prove daunting to the uninitiated.

  This chapter will take you a step closer to writing your own software applications in JavaScript. By explaining the fundamentals, you will be empowered to not only understand what scripts do, but how to reason about problems using JavaScript syntax.

  In this chapter, you will take a close look at JavaScript's type system. All programming languages have a type system. Types literally dictate the type of data stored in a variable or function parameter. Types are typically separated into two categories: primitive and complex types.

  In JavaScript, all primitive data types are immutable. This means that the value cannot be changed in memory. New values can be assigned to a variable, but the underlying data stored in memory cannot be modified directly. This differs from the case in languages such as C++, where values can be directly altered in memory using pointers and helper functions. In JavaScript, when passing a primitive value from one variable to another, the data is copied in memory to the new variable. Therefore, updating one variable does not affect the other.

  Complex data types work differently. They are also known as reference types. Reference types include the Object type and all of its derivatives, such as Array , Date , and Function

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  eBook - ePub

  Beginning JavaScript

  • Jeremy McPeak(Author)
  • 2015(Publication Date)
  • Wrox

    (Publisher)

  In this chapter you have built up knowledge of the fundamentals of JavaScript’s data types and variables and how to use them in operations. In particular, you saw that:

  • JavaScript supports a number of types of data, such as numbers, text, and booleans.
  • Text is represented by strings of characters and is surrounded by quotes. You must match the quotes surrounding strings. Escape characters enable you to include characters in your string that cannot be typed.
  • Variables are JavaScript’s means of storing data, such as numbers and text, in memory so that they can be used again and again in your code.
  • Variable names must not include certain illegal characters, like the percent sign (% ) and the ampersand (& ), or be a reserved word, like with .
  • Before you can give a value to a variable, you must declare its existence to the JavaScript interpreter.
  • JavaScript has the four basic math operators, represented by the symbols plus (+ ), minus (− ), star (* ), and forward slash (/ ). To assign values of a calculation to a variable, you use the equals sign (= ), termed the assignment operator.
  • Operators have different levels of precedence, so multiplication and division will be calculated before addition and subtraction.
  • Strings can be joined, or concatenated, to produce one big string by means of the + operator. When numbers and strings are concatenated with the + operator, JavaScript automatically converts the number into a string.
  • Although JavaScript’s automatic data conversion suits us most of the time, on some occasions you need to force the conversion of data. You saw how parseInt() and parseFloat() can be used to convert strings to numbers. Attempting to convert strings that won’t convert will result in NaN (Not a Number) being returned.
  • Arrays are a special type of variable that can hold more than one piece of data. The data is inserted and accessed by means of a unique index number.

  EXERCISES

  1. Write a JavaScript program to convert degrees centigrade into degrees Fahrenheit, and to write the result to the page in a descriptive sentence. The JavaScript equation for Fahrenheit to centigrade is as follows:

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  eBook - PDF

  JavaScript for Web Warriors

  • Patrick Carey, Sasha Vodnik, Patrick Carey(Authors)
  • 2021(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  CHAPTER 2 WORKING WITH FUNCTIONS, DATA TYPES, AND OPERATORS 48 Working with Data Types Variables can contain many different kinds of values such as the time of day, a dollar amount, or a person’s name. A data type is the specific category of information that a variable contains. The data type determines how much memory is allocated for the data stored in the variable. The data type also governs the kinds of operations that can be performed on the variable. Data types that can be assigned only a single value are called primitive types. JavaScript supports the five primitive data types described in Figure 2-6. The JavaScript language also supports the object data type used for creating a collection of properties. You will learn about the object type in a later chapter. Note DATA TYPE DESCRIPTION number A positive or negative number with or without decimal places, or a number written using exponential notation Boolean A logical value of true or false string Text such as “Hello World!" undefined An unassigned, undeclared, or nonexistent value null An empty value Figure 2-6 Primitive JavaScript data types You might be confused about the distinction between a null value and an undefined value. Null is both a data type and a value. You assign the null value to a variable to indicate that the variable does not contain any data. In contrast, an undefined variable is a variable that has never had a value assigned to it, has not been initialized or does not even exist. For example, a variable that has been declared but not given an initial value is undefined but not null. Many programming languages require that you declare the type of data that a variable contains. Such languages are called strongly typed programming languages. A strongly typed language is also known as statically typed, because data types cannot be changed after they have been declared.

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