Implementation of the String. startsWith and endsWith methods in Javascript, javascriptendswith

Implementation of the String. startsWith and endsWith methods in Javascript, javascriptendswith

StartsWith (anotherString) and endsWith (anotherString) are very useful when operating on the String type. StartsWith determines whether the current string starts with anotherString, while endsWith determines whether it ends. Example:

"abcd".startsWith("ab"); // true"abcd".startsWith("bc"); // false"abcd".endsWith("cd");  // true"abcd".endsWith("e");  // false"a".startsWith("a");   // true"a".endsWith("a");    // true

But unfortunately, Javascript does not contain these two methods, and you can only write them by yourself if necessary. Of course, it is not difficult to write it.

if (typeof String.prototype.startsWith != 'function') { String.prototype.startsWith = function (prefix){  return this.slice(0, prefix.length) === prefix; };}

String. slice () is similar to String. substring (). It obtains a substring. However, it is evaluated that slice is more efficient. The reason that indexOf () is not used here is that indexOf will scan the entire string. If the string is long, indexOf will be inefficient.

if (typeof String.prototype.endsWith != 'function') { String.prototype.endsWith = function(suffix) {  return this.indexOf(suffix, this.length - suffix.length) !== -1; };}

Unlike startsWith, indexOf can be used in endsWith. The reason is that it only scans the last string, and is more efficient than slice because it does not need to copy the string and scan it directly.