Question

我知道如果你想检查是否定义了变量a，你可以这样做

if (typeof a !== 'undefined') {
    // the variable is defined
}

但是如果你想用它来创建一个函数，比如这个

function checkDefined(name) {
    return typeof name !== 'undefined';
}
checkDefined("a");

这不起作用，但如果我必须传递变量名的字符串版本，我怎么能让它工作？

由于

Answer 1

签入全局范围（窗口）：

＆＃13;

var a = 'test';
var b = function() {};

function checkDefined(name) {
    return typeof this[name] !== 'undefined';
}

console.log(checkDefined("a"));
console.log(checkDefined("b"));
console.log(checkDefined("c"));

＆＃13;

如果要检查是否在类对象中声明了变量或函数，则应将新上下文传递给checkDefined方法：

＆＃13;

    function MyCustomClass() {
        this.c = 'test';
    }

    function checkDefined(name) {
        return typeof this[name] !== 'undefined';
    }

    // Create new object of type MyCustomClass
    var myCustomClassObject = new MyCustomClass();

    // In this way you can check if variable/function is defined in object
    console.log(checkDefined.apply(myCustomClassObject, ["a"]));
    console.log(checkDefined.apply(myCustomClassObject, ["b"]));
    console.log(checkDefined.apply(myCustomClassObject, ["c"]));

＆＃13;

apply将立即调用一个函数，让你指定this的值和函数将接收的任何参数

Answer 2

受此answer的启发。我认为您可以尝试使用 eval 返回：

function checkDefined(name) {
  return eval("typeof " + name) !== 'undefined';
}

示例：

var a = 1; checkDefined("a") // true checkDefined(a) // true checkDefined("b") // false

Answer 3

您还应传递需要检查变量是否已定义的对象上下文。如果它的全局传递窗口对象

function checkDefined(name, ref) {
    if(!ref) {
      ref = window;
    }
    return !!ref[name]
}


checkDefined("a"); //false if already not defined


var obj = { a: 1, b:2};
checkDefined("a",obj);//true

Answer 4

局部变量是当前作用域this对象的属性。

const log = output => document.querySelector('pre')
  .innerText += output + '\n'
  

/* In this example, running in the browser, `this` points to `window`, but 
   in other environments would still point to whatever is the global object.
   Bind `this` to the ``checkDefined` so to ensure it keeps the same value
   as where you are calling from. */

const checkDefined = function chkDef(v) {
  return typeof this[v] !== 'undefined'
}.bind(this)

a = 5

log(checkDefined('a'))
log(checkDefined('b'))

/* This is the same basic idea, but we make it a little more portable by
   not binding until right before we use it, so `this` has the correct
   scope. */


unboundCheckDefined = function chkDef(v) {
  return typeof this[v] !== 'undefined'
}
newScope()

function newScope() {
  c = 5
  const checkDefined = 
    unboundCheckDefined.bind(this)
    
  log(checkDefined('a'))
  log(checkDefined('b'))
  log(checkDefined('c'))
}

<pre></pre>

如果你在javascript中有一个字符串，如何定义一个变量？

4 个答案: