PHP's substr_compare in JavaScript

Here’s what our current JavaScript equivalent to PHP's substr_compare looks like.

module.exports = function substr_compare (mainStr, str, offset, length, caseInsensitivity) { // eslint-disable-line camelcase
// discuss at:
// original by: Brett Zamir (
// original by: strcasecmp, strcmp
// example 1: substr_compare("abcde", "bc", 1, 2)
// returns 1: 0
if (!offset && offset !== 0) {
throw new Error('Missing offset for substr_compare()')
if (offset < 0) {
offset = mainStr.length + offset
if (length && length > (mainStr.length - offset)) {
return false
length = length || mainStr.length - offset
mainStr = mainStr.substr(offset, length)
// Should only compare up to the desired length
str = str.substr(0, length)
if (caseInsensitivity) {
// Works as strcasecmp
mainStr = (mainStr + '').toLowerCase()
str = (str + '').toLowerCase()
if (mainStr === str) {
return 0
return (mainStr > str) ? 1 : -1
// Works as strcmp
return ((mainStr === str) ? 0 : ((mainStr > str) ? 1 : -1))
[ View on GitHub | Edit on GitHub | Source on GitHub ]

How to use

You you can install via npm install locutus and require it via require('locutus/php/strings/substr_compare'). You could also require the strings module in full so that you could access strings.substr_compare instead.

If you intend to target the browser, you can then use a module bundler such as Parcel, webpack, Browserify, or rollup.js. This can be important because Locutus allows modern JavaScript in the source files, meaning it may not work in all browsers without a build/transpile step. Locutus does transpile all functions to ES5 before publishing to npm.

A community effort

Not unlike Wikipedia, Locutus is an ongoing community effort. Our philosophy follows The McDonald’s Theory. This means that we don't consider it to be a bad thing that many of our functions are first iterations, which may still have their fair share of issues. We hope that these flaws will inspire others to come up with better ideas.

This way of working also means that we don't offer any production guarantees, and recommend to use Locutus inspiration and learning purposes only.


Please note that these examples are distilled from test cases that automatically verify our functions still work correctly. This could explain some quirky ones.

#codeexpected result
1substr_compare("abcde", "bc", 1, 2)0

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