英文:
Cannot read properties of undefined while testing if undefined (but it is defined)?
问题
console.log(this.variableInQuestion);
this.arrayOfObjects.each(function(thing) {
if (this.variableInQuestion !== undefined && thing.property == this.variableInQuestion) {
...
}
});
英文:
I have the following chunk of Typescript code in a function, with variableInQuestion = 0 being defined outside of the function:
console.log(this.variableInQuestion);
this.arrayOfObjects.each(function(thing) {
if (this.variableInQuestion !== undefined && thing.property == this.variableInQuestion) {
...
}
});
The problem is that the console.log line prints out exactly what I would expect (the default value of 0), but I get an error for the third line saying Cannot read properties of undefined (reading 'variableInQuestion') at [[[location of the first variableInQuestion in the third line]]]. I don't understand why it's giving me a "cannot read properties of undefined" error while I am testing to see if it's undefined.
EDIT: the .each thing is specific to the array of objects in question; that's not the issue. If I run the following...
console.log(this.variableInQuestion);
this.arrayOfObjects.each(function(thing) {
if (0 !== undefined && thing.property == 0) {
...
}
});
It works exactly as I'd expect.
答案1
得分: 1
解决这个问题的方法如下：
variableInQuestion = 0;
myFunction() {
let placeholder = this.variableInQuestion;
this.arrayOfObjects.each(function(thing) {
if (placeholder !== undefined && thing.property == placeholder) {
...
}
});
}
英文:
The way to address this problem is as follows:
variableInQuestion = 0;
myFunction() {
let placeholder = this.variableInQuestion;
this.arrayOfObjects.each(function(thing) {
if (placeholder !== undefined && thing.property == placeholder) {
...
}
});
}
Special thanks to Pointy for identifying the problem!
答案2
得分: 0
如果你使用 console.log 输出你的数据，你会看到：
class X {
variableInQuestion = 123;
arrayOfObjects = [1, 2, 3];
example() {
console.log(this.variableInQuestion);
this.arrayOfObjects.forEach(function (thing) {
console.log({
this: this,
thing: thing,
})
})
}
}
new X().example()
// 123
// { "this": undefined, thing: 1 }
// { "this": undefined, thing: 2 }
// { "this": undefined, thing: 3 }
因为 forEach 不保留 this 值。
要么使用 Lambda 表达式：
this.arrayOfObjects.forEach((thing) => { console.log(this) })
要么保留 this 值：
let that = this;
this.arrayOfObjects.forEach((thing) => { console.log(that) })
或者在 forEach 中提供 thisArg（不知道 .each，但 Array.prototype.forEach 支持这个）：
this.arrayOfObjects.forEach(function(thing){ console.log(this) }, this)
英文:
If you console.log your data, you'll see
class X {
variableInQuestion = 123;
arrayOfObjects = [1, 2, 3];
example() {
console.log(this.variableInQuestion);
this.arrayOfObjects.forEach(function (thing) {
console.log({
this: this,
thing: thing,
})
})
}
}
new X().example()
// 123
// { "this": undefined, thing: 1 }
// { "this": undefined, thing: 2 }
// { "this": undefined, thing: 3 }
because forEach doesn't keep this value.
Either use lambda
this.arrayOfObjects.forEach((thing) => { console.log(this) })
, keep this value
let that = this;
this.arrayOfObjects.forEach((thing) => { console.log(that) })
, or provide thisArg into forEach (dunno about .each but Array.prototype.forEach supports that)
this.arrayOfObjects.forEach(function(thing){ console.log(this) }, this)
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