LANGUAGE » JAVASCRIPT

Variable

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Reference:developer.mozilla.org

Types

Check the type of a variable, for primitive types (if used against class instances, it gives object):

javascript

typeof myVar;  // 'string', 'number', 'boolean', 'object', 'undefined'

Check the type of a variable, for class instances:

javascript

myvar instanceof Date;  // true|false

Casting:

javascript

const flag = Boolean(0);  // !!0 - true for empty lists and objects!

Use the Nullish coalescing operator to fallback a null or undefined variable:

javascript

const errMsg = error.message ?? 'Unknown error';

javascript

var globalVar = 'Globally or function scoped';
let localVar = 'Block scoped';
const localConst = 'Block scoped';

Global variables can be accessed through the window variable.

javascript

window.hasOwnProperty('globalVar');
window.globalVar;