The `toString()` Function in JavaScript
Most JavaScript objects and primitive values have a toString() function that converts
the value to a string. Many built-in methods use toString() under the hood, like the browser's alert() function.
Primitives
JavaScript number primitives have a toString() function that converts the number to a string.
This is one of the most common uses for toString():
const num = 42;
num.toString(); // '42'
typeof num.toString(); // 'string'
// Can also use `toString()` on a number literal as long as you
// use parentheses.
(42).toString(); // '42'All primitive values except null and undefined have a toString() function: strings, numbers, booleans, BigInts, and
symbols.
// String:
'Hello'.toString(); // 'Hello'
// Number:
(42).toString(); // '42'
// Boolean:
true.toString(); // 'true'
// BigInt:
42n.toString(); // '42'
// Symbol:
Symbol('test').toString(); // 'Symbol(test)'The important takeaway is that it is safe to call toString() on an arbitrary JavaScript value
as long as that value is not null or undefined. The easiest way to check is to use == null:
the most common use for == is that v == null is shorthand for v === null || v === undefined.
if (v != null) {
// Won't throw an error, unless you wrote a custom `toString()` that throws
v.toString();
}Objects
The Object class in JavaScript is the base class for all objects, and it has a simple toString()
method that usually prints [object Object]:
// Equivalent to `const obj = {};`
const obj = new Object();
obj.toString(); // '[object Object]'The [object Object] output is often confusing to beginners because they want to see the object's
keys and values. You can loop over the object's keys and values yourself, but the easiest one-liner is to use JSON.stringify().
const obj = { name: 'Jean-Luc Picard', rank: 'Captain' };
// '{"name":"Jean-Luc Picard","rank":"Captain"}'
console.log(JSON.stringify(obj));If you define a JavaScript class, you can overwrite the toString()
function to return whatever you want:
class MyClass {
toString() {
return 'Hello, World!';
}
}
const obj = new MyClass();
obj.toString(); // 'Hello, World!'toString() Parameters
Some toString() functions take parameters, most notably numbers and Node.js buffers.
The toString() function for JavaScript numbers takes a radix parameter that defines the base of
the numeral system. In other words, num.toString(2) converts the number to a binary number string, num.toString(10) converts the number to a base-10 string,
and num.toString(16) converts the number to a hexadecimal string.
(3).toString(2); // '11'
(42).toString(10); // '42'
(29).toString(16); // '1d'The Node.js buffer toString() function takes an encoding
parameter that is usually one of 'utf8', 'hex', or 'base64'. This determines how the raw data in
the buffer is encoded.
const fs = require('fs');
const buf = fs.readFileSync('./package.json');
buf.toString('utf8'); // '{ "name": "masteringjs.io", ...}'