Variables accept letters, numbers, underscores, and dollar signs, though they cannot start with numbers. Reserved keywords cannot serve as variable names. The JavaScript community favors lower camelCase naming conventions like firstName or lastName.
Both single and double quotes create valid strings. The typeof operator identifies variable types. Strings support operations including concatenation with the + operator, length measurement, index access, and the replace() method.
Numeric types include integers, decimals, and negative values. Standard mathematical operators apply, along with the Math object for operations like Math.random(), Math.ceil(), and Math.round(). The += operator works for incremental changes.
Comparison operators include == (equal value), === (identical type and value), != (not equal), and !== (not equal value or type).
Both represent absent values. “Undefined” appears when variables lack assignment, while “null” typically indicates previously held values that were reset.
Conventionally use snake_case naming. Functions accept parameters, execute code blocks, and return values using the return keyword.
Ordered collections supporting operations: push() (append), pop() (remove last), shift() (remove first), unshift() (prepend).
Key-value pair collections accessed via bracket notation or dot notation. Methods within objects use the this keyword to reference properties.
Browser actions trigger events like click, change, mouseover, and keydown. The event object provides details about triggered actions, including keyCode for keyboard events.
Select Boxes: Access selected options via selectedIndex or direct .value property.
Radio Buttons: Use getElementsByName() to retrieve radio button arrays, then iterate to identify checked items.
Checkboxes: Similar to radio buttons, retrieve arrays and loop through checked elements.
Change Events: The onchange event triggers when form elements change state.
jQuery simplifies JavaScript syntax. Almost all jQuery statements begin with $, using CSS-like selectors: $(".class"), $("element"), $("#id").
html() gets/sets inner HTMLattr() accesses element attributesval() retrieves form field values
- Radio buttons:
$("input[name='gender']:checked").val()
- Checkboxes:
$("input[name='interest']:checked")
$.each() loops through jQuery collections
click(), change() bind functions to events$(document).ready() ensures DOM loads before executionon() method handles single or multiple events
Functions passed as arguments execute after parent function completion. Essential for handling asynchronous operations like API calls with delays.
try/catch blocks manage errors. The throw statement creates custom exceptions.
Objects group related variables and methods, simulating namespace functionality: var module = { property: value, method: function() {} }
JSON.stringify() converts objects to strings; JSON.parse() reverses the process.
- localStorage: Persists after browser closes
- sessionStorage: Clears when session ends
Both accept string values only; use JSON.stringify() for objects.
Create requests with new XMLHttpRequest(). Monitor state changes via onreadystatechange. Success occurs when readyState == 4 and status == 200.
Simplified syntax using $.ajax() with parameters: url, type, dataType, success, and error callbacks.