Day 0: Data Types

Data Types

JavaScript's Data Types

The latest ECMAScript standard defines seven data types:

  • A primitive value or data type is data that is not an object and has no methods. All primitives are immutable, meaning they cannot be changed. There are six primitive types:
    • Number
    • String
    • Boolean
    • Symbol
    • Null
    • Undefined
  • The seventh data type is Object

Number Data Type

According to the ECMAScript standard, all numbers are double-precision 64-bit binary format IEEE 754-2008, meaning there is no specific type for integers.

Maximum Value for a Number

The MAX_VALUE property has a value of approximately . Values larger than Number.MAX_VALUE are represented as Infinity.

Minimum Value for a Number

The MIN_VALUE property is the smallest positive value of the Number type closest to , not the most negative number, that JavaScript can represent. MIN_VALUE has a value of approximately . Values smaller than Number.MIN_VALUE ("underflow values") are converted to .

Symbolic Numbers

There are three symbolic number values:

  • Infinity: This is any number divided by , or an attempt to multiply Number.MAX_VALUE by an integer .
  • -Infinity: This is any number divided by , or an attempt to multiply Number.MAX_VALUE by an integer .
  • NaN: This stands for "Not-a-Number" and denotes an unrepresentable value (i.e., ).

The isSafeInteger Method

A safe integer is an integer that:

  • Can be exactly represented as an IEEE-754 double precision number, and
  • Whose IEEE-754 representation cannot be the result of rounding any other integer to fit the IEEE-754 representation.

The Number.isSafeInteger() method determines whether the provided value is a number that is a safe integer.

Maximum Safe Integer

The Number.MAX_SAFE_INTEGER constant has a value of , or .

Minimum Safe Integer

The Number.MIN_SAFE_INTEGER constant has a value of , or .

EXAMPLE
Run the code below to learn more about the Number type.

var var1 = 1;
var var2 = 0;
var var3 = -0;

console.log("1 / 0 = " + var1 / var2); 
console.log("1 / -0 = " + var1 / var3); 
console.log("MAX_VALUE: " + Number.MAX_VALUE);
console.log("MAX_VALUE + 1 = " + Number.MAX_VALUE * 2);
console.log("MIN_VALUE = " + Number.MIN_VALUE);
console.log("MIN_VALUE - 1 = " + Number.MIN_VALUE - 1);
console.log("MAX_SAFE_INTEGER = " + Number.MAX_SAFE_INTEGER);
console.log("MIN_SAFE_INTEGER = " + Number.MIN_SAFE_INTEGER);
console.log("SquareRoot(-1) = " + Math.sqrt(-1));
console.log("isSafeInteger(100) = " + Number.isSafeInteger(100));
Run
Output

String Data Type

A string value is a chain of zero or more Unicode characters (i.e., letters, digits, and punctuation marks) that we use to represent text. We include string literals in our scripts by enclosing them in single (') or double (") quotation marks. Double quotation marks can be contained in strings surrounded by single quotation marks (e.g., '"' evaluates to "), and single quotation marks can be contained in strings surrounded by double quotation marks (e.g., "'" evaluates to '). The following are examples of strings:

EXAMPLE
Run the code below to learn more about the String type.

var firstString = "Hello, There."; var secondString = "How're you?"; var thirdString = "c"; var fourthString = '"Wow!!!", she shouted.'; console.log(firstString); console.log(secondString); console.log(thirdString); console.log(fourthString);
Run
Output

Notice that JavaScript does not have a type to represent a single character. To represent a single character in JavaScript, you create a string that consists of only one character. A string that contains zero characters ("") is an empty (zero-length) string.

Unlike in languages like C, JavaScript strings are immutable. This means that once a string is created, it is not possible to modify it. However, it is still possible to create another string based on an operation on the original string. For example:

  • A substring of the original by picking individual letters or using String.substr().
  • A concatenation of two strings using the concatenation operator (+) or String.concat().

Boolean Data Type

A boolean represents a logical entity and can have one of two literal values: true, and false.

Symbol Data Type

Symbols are new to JavaScript in ECMAScript Edition 6. A Symbol is a unique and immutable primitive value and may be used as the key of an Object property.

Null Data Type

The null data type is an internal type that has only one value: null. This is a primitive value that represents the absence of any object value. A variable that contains null contains no valid number, string, boolean, array, or object. You can erase the contents of a variable (without deleting the variable) by assigning it the null value.

Undefined Data Type

The undefined value is returned when you use an object property that does not exist, or a variable that has been declared, but has never had a value assigned to it.

The typeof Operator

As demonstrated in some of the code examples above, we can use the typeof operator to determine the type associated with a variable's current value:

EXAMPLE
Run the code below to learn more about typeof.

// Number Data Type: var firstVar = 1.5e5; // String Data Type: var secondVar = 'Hello'; // Boolean Data Type: var thirdVar = true; // Symbol Data Type: var fourthVar = Symbol("some Symbol variable"); // Null Object: var fifthVar = null; // Undefined Data Type: var sixthVar; // Object: var seventhVar = {a: 1, b: 2}; // NaN (It is a Number): var eighthVar = Math.sqrt(-1); console.log(firstVar + " is a " + typeof firstVar); console.log(secondVar + " is a " + typeof secondVar); console.log(thirdVar + " is a " + typeof thirdVar); console.log(fourthVar.toString() + " is a " + typeof fourthVar); console.log(fifthVar + " is an " + typeof fifthVar); console.log(sixthVar + " is an " + typeof sixthVar); console.log(seventhVar + " is an " + typeof seventhVar); console.log(eighthVar + " is a " + typeof eighthVar);
Run
Output

Variables

Dynamic Typing

JavaScript is a loosely typed or dynamic language, meaning you don't need to declare a variable's type ahead of time and the language autmatically determines a variable's type while the program is being processed. That also means that you can reassign a single variable to reference different types. For example:

EXAMPLE
Run the code below to learn more about dynamic typing.

function print() { console.log( "someVariable(" + someVariable + ") is a " + typeof someVariable ); // Note: 'typeof' is explained later in this tutorial. } // Declare someVariable and initialize it to the number '5': var someVariable = 5; print(someVariable); // Assign the string "Hello" to someVariable: var someVariable = "Hello"; print(someVariable); // Assign the boolean value 'true' to someVariable: var someVariable = true; print(someVariable);
Run
Output

Naming

JavaScript is a case-sensitive language, meaning that a variable name such as myVariable is different from the variable name myvariable. Variable names can be of any length, and the rules for creating legal variable names are as follows:

  • The first character must be either an ASCII letter (uppercase or lowercase) or an underscore (_). Note that a number cannot be used as the first character.
  • Subsequent characters can be ASCII letters, underscores, or digits (e.g., the numbers through ).
  • The variable name must not be a reserved word.

The code below declares some valid variable names:

var _daysCount
var MinimumCost
var page10
var Total_elements

The following declarations are invalid variable names and will not compile:

// This will produce "SyntaxError: Unexpected number"
var 10Students

// This will produce "SyntaxError: Unexpected token &"
var First&Second

Declaration and Initialization

The first time a variable appears in your script is considered its declaration. The first mention of the variable sets it up in memory, and the name allows you to refer back to it in your subsequent lines of code. You should declare variables using the var keyword before using them. If you do not initialize a variable that was declared using the var keyword, it automatically takes on the value undefined.

EXAMPLE
Consider the following code:

// Declare firstVar: var firstVar; // Initialize firstVar: firstVar = 1; // Declare and Initialize secondVar: var secondVar = "String"; // Declare thirdVar and fourthVar: var thirdVar, fourthVar; // Initialize thirdVar: thirdVar = true; // Initialize fourthVar: fourthVar = null; // Declare and Initialize fifthVar and sixthVar: var fifthVar = 1.01, sixthVar = "Sixth"; // Declare seventhVar: var seventhVar; console.log(firstVar); console.log(secondVar); console.log(thirdVar); console.log(fourthVar); console.log(fifthVar); console.log(sixthVar); console.log(seventhVar);
Run
Output

Coercion

In JavaScript, you can perform operations on values of different types without raising an exception. The JavaScript interpreter implicitly converts, or coerces, one of the data types to that of the other, then performs the operation. The rules for coercion of string, number, or boolean values are as follows:

  • If you add a number and a string, the number is coerced to a string.
  • If you add a boolean and a string, the boolean is coerced to a string.
  • If you add a number and a boolean, the boolean is coerced to a number.

EXAMPLE
Run the code below to learn more about type coercion.

function print(name, variable) { console.log( name + "(" + variable + ") is a " + typeof variable ); } var someNumber = 10; var someString = "Ten"; var someBoolean = false; var sumOfNumberAndString = someNumber + someString; var sumOfBooleanAndString = someBoolean + someString; var sumOfNumberAndBoolean = someNumber + someBoolean; print("sumOfNumberAndString", sumOfNumberAndString); print("sumOfBooleanAndString", sumOfBooleanAndString); print("sumOfNumberAndBoolean", sumOfNumberAndBoolean);
Run
Output

 

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