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Syntax
typeof operand
Basic Examples
// Numbers
typeof 37 === 'number';
typeof 3.14 === 'number';
typeof(42) === 'number';
typeof Math.LN2 === 'number';
typeof Infinity === 'number';
typeof NaN === 'number'; // Despite being "Not-A-Number"
typeof Number('1') === 'number'; // Number tries to parse things into numbers
// Strings
typeof '' === 'string';
typeof 'bla' === 'string';
typeof `template literal` === 'string';
typeof '1' === 'string'; // note that a number within a string is still typeof string
typeof (typeof 1) === 'string'; // typeof always returns a string
typeof String(1) === 'string'; // String converts anything into a string, safer than toString
// Booleans
typeof true === 'boolean';
typeof false === 'boolean';
typeof Boolean(1) === 'boolean'; // Boolean() will convert values based on if they're truthy or falsy
typeof !!(1) === 'boolean'; // two calls of the ! (logical NOT) operator are equivalent to Boolean()
// Symbols
typeof Symbol() === 'symbol'
typeof Symbol('foo') === 'symbol'
typeof Symbol.iterator === 'symbol'
// Undefined
typeof undefined === 'undefined';
typeof declaredButUndefinedVariable === 'undefined';
typeof undeclaredVariable === 'undefined';
// Objects
typeof {a: 1} === 'object';
// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === 'object';
typeof new Date() === 'object';
typeof /regex/ === 'object'; // See Regular expressions section for historical results
// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean(true) === 'object';
typeof new Number(1) === 'object';
typeof new String('abc') === 'object';
// Functions
typeof function() {} === 'function';
typeof class C {} === 'function';
typeof Math.sin === 'function';
typeof Operator with Object Properties
// using Object Literals
var user = {
emailId : 'ramesh@gmail.com',
age : 29,
getFullName : function (){
return user.firstName + " " + user.lastName;
}
};
function isUndefined(user) {
if(typeof user.firstName === 'undefined'){
console.log("User first name is undefined");
}
if(typeof user.lastName === 'undefined'){
console.log("User last name is undefined");
}
}
isUndefined(user);
User first name is undefined
User last name is undefined
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