Check if a variable is a string in JavaScript
javascript, string
Solution
You can use `typeof` operator:
var booleanValue = true;
var numericalValue = 354;
var stringValue = "This is a String";
var stringObject = new String("This is a String Object");
console.log(typeof booleanValue) // displays "boolean"
console.log(typeof numericalValue) // displays "number"
console.log(typeof stringValue) // displays "string"
console.log(typeof stringObject) // displays "object"
Example from this webpage. (Example was slightly modified though).
This won't work as expected in the case of strings created with `new String()`, but this is seldom used and recommended against[1][2]. See the other answers for how to handle these, if you so desire.
// Test this approach:
let isString = value => typeof value === 'string';
let falseCases = [
[ 'null', null ],
[ 'undefined', undefined ],
[ 'object', { a: 1, b: 2 } ],
[ 'array', [ 1, 2, 3 ] ],
[ 'number', 123 ],
[ 'zero', 0 ],
[ 'RegExp', new RegExp('hello') ],
[ 'number with valueOf returning string', Object.assign(10, { valueOf: () => 'abc' }) ],
[ 'object pretending to be string', { constructor: String } ]
];
let trueCases = [
[ 'empty literal string', '' ],
[ 'unicode string literal', String.fromCharCode(10000) ],
[ 'empty boxed string', new String('') ],
[ 'unicode boxed string', new String(String.fromCharCode(10000)) ],
[ 'string with overwritten "constructor"', Object.assign('hi', { constructor: Array }) ],
[ 'string with overwritten "toString"', Object.assign('hi', { toString: 123 }) ],
[ 'string with overwritten "valueOf"', Object.assign('hi', { valueOf: 123 }) ],
[ 'string with overwritten "constructor"', Object.assign('hi', { constructor: RegExp }) ],
[ 'proxied string', new Proxy(new String('hello'), {}) ],
];
console.log('NEGATIVE TESTS:');
for (let [ name, val ] of falseCases) {
console.log(`Test ${name}:\n Expect: false\n Got: ${isString(val)}`);
}
console.log('\nPOSITIVE TESTS:');
for (let [ name, val ] of trueCases) {
console.log(`Test ${name}:\n Expect: true\n Got: ${isString(val)}`);
}
- The Google JavaScript Style Guide says to never use primitive object wrappers.
- Douglas Crockford recommended that primitive object wrappers be deprecated.
Problem
How can I determine whether a variable is a string or something else in JavaScript?