startsWith()
方法用来判断当前字符串是否是以另外一个给定的子字符串“开头”的,根据判断结果返回 true
或 false
。
str.startsWith(searchString [, position])
;
searchString
position
str
中搜索 searchString
的开始位置,默认值为 0,也就是真正的字符串开头处。var str = "To be, or not to be, that is the question.";
alert(str.startsWith("To be")); // true
alert(str.startsWith("not to be")); // false
alert(str.startsWith("not to be", 10)); // true
/*! http://mths.be/startswith v0.2.0 by @mathias */
if (!String.prototype.startsWith) {
(function() {
'use strict'; // needed to support `apply`/`call` with `undefined`/`null`
var defineProperty = (function() {
// IE 8 only supports `Object.defineProperty` on DOM elements
try {
var object = {};
var $defineProperty = Object.defineProperty;
var result = $defineProperty(object, object, object) && $defineProperty;
} catch(error) {}
return result;
}());
var toString = {}.toString;
var startsWith = function(search) {
if (this == null) {
throw TypeError();
}
var string = String(this);
if (search && toString.call(search) == '[object RegExp]') {
throw TypeError();
}
var stringLength = string.length;
var searchString = String(search);
var searchLength = searchString.length;
var position = arguments.length > 1 ? arguments[1] : undefined;
// `ToInteger`
var pos = position ? Number(position) : 0;
if (pos != pos) { // better `isNaN`
pos = 0;
}
var start = Math.min(Math.max(pos, 0), stringLength);
// Avoid the `indexOf` call if no match is possible
if (searchLength + start > stringLength) {
return false;
}
var index = -1;
while (++index < searchLength) {
if (string.charCodeAt(start + index) != searchString.charCodeAt(index)) {
return false;
}
}
return true;
};
if (defineProperty) {
defineProperty(String.prototype, 'startsWith', {
'value': startsWith,
'configurable': true,
'writable': true
});
} else {
String.prototype.startsWith = startsWith;
}
}());
}
Feature | Chrome | Firefox (Gecko) | Edge | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 41 | 17 (17) | (Yes) | 未实现 | 28 | 9 |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | 未实现 | 36 | 17.0 (17) | 未实现 | 未实现 | 未实现 |
请注意,尽管MSDN文档有清晰地指示这个方法 (https://msdn.microsoft.com/en-us/library/mt146831(v=vs.94).aspx) 不支持Internet Explorer,,但是这个方法似乎会在很久之前的Internet Explorer 8下生效。