使用常量作为 Javascript 关联数组的索引
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Using constants as indices for Javascript Associative Arrays
提问by Edan Maor
I'm looking to create an associative array in JS, but use constants defined as part of the class as indices.
我希望在 JS 中创建一个关联数组，但使用定义为类的一部分的常量作为索引。
The reason I want this is so that users of the class can use the constants (which define events) to trigger actions.
我想要这个的原因是为了让类的用户可以使用常量（定义事件）来触发操作。
Some code to illustrate:
一些代码来说明：
STATE_NORMAL = 0;
STATE_NEW_TASK_ADDED = 0;
this.curr_state = STATE_NEW_TASK_ADDED;
this.state_machine = {
/* Prototype:
STATE_NAME: {
EVENT_NAME: {
"next_state": new_state_name,
"action": func
}
}
*/
STATE_NEW_TASK_ADDED : { // I'd like this to be a constant
this.EVENT_NEW_TASK_ADDED_AJAX : {
"next_state": STATE_NEW_TASK_ADDED,
"action" : function() {console.log("new task added");},
}
}
}
// Public data members.
// These define the various events that can happen.
this.EVENT_NEW_TASK_ADDED_AJAX = 0;
this.EVENT_NEW_TASK_ADDED_AJAX = 1;
I'm having trouble getting this to work. I'm not too great with JS, but it looks like no matter what I do, the array gets defined with strings and not constants. Is there any way to force the array to use the constants?
我很难让它正常工作。我对 JS 不太熟悉，但看起来无论我做什么，数组都是用字符串而不是常量定义的。有没有办法强制数组使用常量？
Thanks!
谢谢！
回答by Ashe
See @Kristian's answer below re: ES6/modern JavaScript, which has new syntax to make this possible.
请参阅下面的@Kristian 回答：ES6/现代 JavaScript，它具有使这成为可能的新语法。
The below is my original answer, from the pre-modern age.
以下是我的原始答案，来自前现代时代。
The problem here, actually, is that you can't use a value for the key part when you're defining an object literally.
实际上，这里的问题是，当您按字面意思定义对象时，不能为关键部分使用值。
That is to say, this uses the constant values as expected:
也就是说，这按预期使用常量值：
var CONSTANT_A = 0, CONSTANT_B = 1;
var state_machine = {};
state_machine[CONSTANT_A] = "A";
state_machine[CONSTANT_B] = "B";
console.log(state_machine[0]); // => A
console.log(state_machine[1]); // => BBut this won't work as expected, instead using the string CONSTANT_Aas key:
但这不会按预期工作，而是使用字符串CONSTANT_A作为键：
var CONSTANT_A = 0, CONSTANT_B = 1;
var state_machine = {
CONSTANT_A: "A",
CONSTANT_B: "B",
};
console.log(state_machine[0]); // => undefined
console.log(state_machine["CONSTANT_A"]); // => A
console.log(state_machine.CONSTANT_A); // => AJavaScript has a shorthand to define object literals where you can omit the double-quotes around keys. Expressions can't be used, so CONSTANT_Awon't be evaluated.
JavaScript 有一个速记来定义对象字面量，您可以在其中省略键周围的双引号。表达式不能被使用，所以CONSTANT_A不会被评估。
回答by Kristian
In ES6 you can use computed values for object keys.
在 ES6 中，您可以为对象键使用计算值。
var CONSTANT_A = 0, CONSTANT_B = 1
var state_machine = {
[CONSTANT_A]: function () {
return 'a'
},
[CONSTANT_B]: function () {
return 'b'
}
};
console.log(state_machine)This does not work in IE 11 nor in safari browsers: https://kangax.github.io/compat-table/es6/#test-object_literal_extensions_computed_properties
这在 IE 11 和 safari 浏览器中都不起作用：https: //kangax.github.io/compat-table/es6/#test-object_literal_extensions_computed_properties
