I want TypeScript to avoid undefined index access

In the code below, the object may be 'undefined'.ts(2532)
Also, there is an error in the test[i].v part, but I don't know the cause at all.
Is there any way to avoid writing code?

//test.ts
typeTestArray={
  v:string;
}
let test —TestArray[]=[];
for(leti=0;i<test.length;i++){
  if(test[i]&"1.1" == test[i].v){
    console.log(test);
  }
}

typescript

2022-10-05 01:00

1 Answers

(Assume "noUncheckedIndexedAccess": true in tsconfig.json and so on.Otherwise, you won't get a question error.)

The type Narrowing is a variable; you cannot narrow down an expression like test[i].

Replace with variable

Therefore, it is possible to substitute a variable before handling it.

for(leti=0;i<test.length;i++){
    // Here's here
    const val = test [i]

    // I checked on the left, so the val on the right is narrowed down to undefined.
    if(val&"1.1" == val.v){
        console.log(val)
    }
}

Optional Chain

The option chain allows you to write checks compactly, and the TypeScript type is fine.

for(leti=0;i<test.length;i++){
    // The result of `test[i]?.v` may be undefined.
    // In this case, it will be false compared to 1.1 so there is no problem.
    if("1.1" == test[i]?.v){
        console.log(test[i])
    }
}

Null Assertion Operator (`!`)

If you are certain that it is not undefined, you can skip the check using Non-Null Assertion Operator.

(In the case of self-made code, it's OK, but in the JavaScript world, there are objects whose content changes just by reading properties, so I can't say for sure if I just look at this part of the code.)


for(leti=0;i<test.length;i++){
    // I know that test[i] is not undefined.
    // Tell TypeScript.Runtime error if incorrect.
    if("1.1" == test[i]!.v) {
        console.log(test[i])
    }
}

Another Loop Method

Depending on the syntax, you don't have to check it yourself.

// "for of" loop
for (const value of test) {
    if("1.1" == val.v) {
        console.log(val)
    }
}

// forEach
test.forEach(val)=>{
    if("1.1" == val.v) {
        console.log(val)
    }
})

2022-10-05 01:00

If you have any answers or tips

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