JS Cheatsheet

js-cheatsheet.md

JS Cheat Sheet

A selection of JavaScript snippets for your quick perusal when you are dashing between billion-dollar ideas.

Trivial expressions

blah

A plain word refers to a variable in the current environment.

"blah"

A quoted word is a string, a value containing piece of text.

12

A number value.

true

Boolean (yes/no) value. true for yes, false for no.

Operator expressions

a + b

Binary operator applied to two values. + to add, - to subtract, * to multiply, / to divide.

(a + b) * c

Parenthesis for explicit grouping.

a < b

: Comparison operators ==, != (not equal), <, >, <= (less or equal), >=.

a = b

Assignment, set variable a to value b. Not to be confused with == comparison. a += b is a shorthand for a = a + b, also for -= etc.

a && b

Logical operators — && for AND, || for OR.

-a

Unary (one-operand) operator. - to negate (make negative)

!a

Unary (one-operand) operator. ! for boolean negation.

!!a

Using two ! operators to work out if something is Truthey.

Composite expressions

a[b]

Subscript, fetch the field named by b from value a.

a.x

Shorthand for a["x"].

a(b)

Function call. Call the function value a with b as argument. Zero or more argument expressions can be given, separated by spaces. a(1, 2, 3, 4)

a.x(b)

Method call. Call the function found in field x of value a, and pass a as the this argument.

[1, 2, 3, 4]

Array value with zero or more elements.

{a: 1, b: 2}

Object value with zero or more name: value field definitions.

function(arg1, arg2) { 
  /* ... body ... */ 
}

Function value. Zero or more argument names. Any statements may appear in body.

Statements

a;

Any expression, followed by a semicolon, is a statement.

var a = b;

Variable definition. The variable with name a is defined and given value b. Value is optional. var a; sets a to undefined.

function foo(arg1, arg2) { 
  // code
}

Function definition. Defines variable foo to have a function value. Zero or more arguments, any statements may appear in body.

if (a) { 
  // code if expression true
} else { 
  // code if expression false
}

Conditional statement. If value a is true, the first statement, otherwise the else statement executes. Else part may be left off.

if (a) {
  // code
} else if (b) {
  // code
} else {
  // code
}

Chaining conditional expressions.

while (a) { 
  // code to execute in the loop
}

A while loop. The loop body statement will be executed as long as a produces a true value.

for (var a = 0; a < 10; a++) { 
  // code 
}

Example for-loop statement. var a = 1 initializes the loop, a < 10 checks whether it has ended yet, and a++ moves to the next step by incrementing the counter a.

return a;

Only valid inside a function or after a control flow struction like if. Returns value a as the result of the function call.

DOM

document

An object-orientated representation of the HTML that was read by the browser.

var greeting = document.getElementById("hello")

Search for an element with the id of hello in the document and assign to the variable greeting (can be named anything.

var divs = document.getElementsByTagName("div")

Search for all div tags within the document using the html tag.