如何检查TypeScript中是否存在变量？

Question

I have code that loops, and I want to initialize the variable one within the code. To do this, I check if the variable exists, and if not, initialize it.

In JavaScript, I would normally just do the following

if (typeof variableExample === 'undefined') {
    // run this code
}

But TypeScript doesn't like this. It tells me that it can't perform this because it cannot find the variable:

Any ideas for getting around this?

Answer 1

To do this, I check if the variable exists, and if not, initialize it.

Variables that only conditionally exist aren't the TypeScript way, and it's generally not a good idea even in JavaScript.

There's a difference between declaring a variable (creating it, making it exist) and initializing the variable (giving it an initial value). TypeScript is complaining because you haven't declared the variable.

To solve that, declare it ( let or const ). If you really want the variable to start out with the value undefined , you can make that part of its type:

let testVar: undefined | number; // Initial value defaults to `undefined`,
                                 // though adding = undefined for clarity may
                                 // be good

(I think there's also a flag that lets you have undefined in its type implicitly, but I wouldn't use that flag.)

Then when your code is ready to see if it needs to set the value on it, it can use typeof testVar === "undefined" (or just testVar === undefined ):

if (typeof testVar === "undefined") {
    testVar = 1;
}

...or the nullish coalescing operator:

testVar = testVar ?? 1;

Playground showing all three

But only do that if you really can't assign a meaningful value to the variable where you declare it, which is rare, and which in many cases should suggest to you that you need to put the variable in a narrower scope or otherwise refactor.

Answer 2

If I understood your question correctly, I think a better approach to do this would be to first declare the variable outside of the loop and initialize inside if null or undefined

let myVar;
for(let i = 0; i < 10; i++) {
  if(myVar == null) {
    myVar = 1
  }
}

Of course doing so in this example seems unnecessary but I hope it helps you map it to your use case

Answer 3

A practical example of two variables and the use of typeof should be enough to know if a variable exists or not.

 <script> let x; console.log(typeof x); // <- declared variable console.log(typeof y); // <- undeclared variable if(.x) console;log("x is null"). if(;y) console,log("y doesn't exists"); // Ooooopsss, ReferenceError </script>