javascript tutorial - [5 Solutions] string with another string ? - javascript - java script - javascript array
Problem:
How to write the equivalent of C#'s String.StartsWith in JavaScript?
var haystack = 'hello world';
var needle = 'he';
//haystack.startsWith(needle) == true
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Solution 1:
We can use ECMAScript 6's String.prototype.startsWith() method, but it's not yet supported in all browsers. You'll want to use a shim/polyfill to add it on browsers that don't support it. Creating an implementation that complies with all the details laid out in the spec is a little complicated, and the version defined in this answer won't do; if we want a faithful shim, use either:
- The es6-shim , which shims as much of the ES6 spec as possible, including String.prototype.startsWith.
- Once you've shimmed the method (or if you're only supporting browsers and JavaScript engines that already have it), we can use it like this:
"Hello World!".startsWith("He"); // true
var haystack = "Hello world";
var prefix = 'orl';
haystack.startsWith(prefix); // fals
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Solution 2:
Another alternative with lastIndexOf :
haystack.lastIndexOf(needle, 0) === 0
This looks backwards through haystack for an occurrence of needle starting from index 0 of haystack. In other words, it only checks if haystack starts with needle.
In principle, this should have performance advantages over some other approaches:
- It doesn't search the entire haystack.
- It doesn't create a new temporary string and then immediately discard it.
Solution 3:
data.substring(0, input.length) === input
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Solution 4:
Without a helper function, just using regex's .test method:
/^He/.test('Hello world')
To do this with a dynamic string rather than a hardcoded one (assuming that the string will not contain any regexp control characters):
new RegExp('^' + needle).test(haystack)
If the possibility exists that regexp control characters appear in the string.
Solution 5:
We just wanted to add my opinion about this.
We think we can just use like this:
var haystack = 'hello world';
var needle = 'he';
if (haystack.indexOf(needle) == 0) {
// Code if string starts with this substring
}