1.2 Variables

Variable Declaration and Assignment

Variables are declared using the var keyword. JS is dynamically typed (i.e. the type of variables can change at runtime). For example, you can assign an integer to a variable and along the way assign a string to the same variable (thus changing the type of the variable to string type). typeof is a special operator for getting the type of a variable (in string format).

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var a = 5;
console.log(a + " " + typeof a); //5 number
a = "hello";
console.log(a + " " + typeof a); //hello string

//declare multiple variables
var n1, n2;

Datatypes

JS has 6 primitive data types: undefined, null, number, string, boolean, and symbol. You can choose to enclose strings with single quotes (') or double quotes (") - just use the matching type of quotes. There is also the object type for complex type. You can also assign functions to a variable.

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typeof 1; //number
typeof 1.12; //number
typeof "Tom"; //string
typeof true; //boolean
String(1234); //"1234"
typeof String(1234); //string

typeof { name: "John" }; //object
typeof [1, 2, 3]; //object
typeof new Date(); //object

var f = function () {};
typeof f; //function

Undefined vs Not Defined

Other than assigning proper values to a variable, there are also other special values (null and undefined) in Javascript. Note that undefined is not the same as not defined. A variable takes on the value of undefined if it is declared but not assigned any value. Whereas, if you try to access a variable that is not declared, an error is thrown indicating that the variable is not defined (or not declared).

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var c = null;
console.log(c + " " + typeof c); //null object

var b; //same as b = undefined
console.log(b + " " + typeof b); //undefined undefined
console.log(d + " " + typeof d); //Error: d is not defined

undefined + "abc"; //"undefinedabc"
undefined + "1"; //undefined1

typeof (undefined + "abc"); //string

Auto Type Conversion

Javascript does auto type conversion when you assign a value to a variable. It tries its best to determine the type of your variable/expression. JS also performs type coercion when an expression contains multiple types. You are allowed to concatenate different data types (e.g. a string with a number) like in the case of Java.

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1 / 10; //0.1
0.9 + 0.1; //1

1 / "10"; //0.1
"10" * "2"; //20

1 + "10"; //"110"
"10" + 1; //"101"
+"10" + 1; //11
"123" - 3; //120

1 + true; //2
true + "hello"; //"truehello"

Number("100") + 200; //300
Number(" "); //0 (same as Number(""))

Like with most programming languages, when handling numeric arithmetic, NaN (Not a Number) and Infinity cases are usually quite weird.

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undefined + 1; //NaN
typeof NaN; // number

1 / 0; //Infinity
typeof (1 / 0); //number
typeof Infinity; //number

1 / 0 - 1; //Infinity
Infinity - Infinity; //NaN
Infinity + Infinity; //Infinity
Infinity * Infinity; //Infinity
Infinity / Infinity; //NaN