从 function 返回未定义

Question

I have a function in a class definition that returns undefined . Here is the class:

class Lexer {
  // constructor and another method here
  make_tokens() {
    var tokens = [];
    // checking character values here
    console.log(tokens); 
    // Outputs [ Token { type: 'PLUS', value: undefined }, Token { type: 'PLUS', value: undefined } ] if I enter ++
    return tokens, null; // returns undefined, null
  }
  // make_numbers() goes here
}

When I searched for answers, I mainly got returning from asynchronous functions but my function is not asynchronous.

I do not know what the problem is.

(sorry if I didn't structure it well but I am new to StackOverflow and I don't know how to structure a question properly)

Answer 1

i don't think you could return value that way.

 class Lexer { // constructor and another method here make_tokens() { var tokens = []; // checking character values here console.log(tokens); // Outputs [ Token { type: 'PLUS', value: undefined }, Token { type: 'PLUS', value: undefined } ] if I enter ++ return [tokens, null]; // returns undefined, null } // make_numbers() goes here } const lexer = new Lexer(); console.log(lexer.make_tokens());

Answer 2

You can't return multiple values like that.

I think you end up just returning the last value which in this case is null .

Why do you need to return null aswell?

If you just return tokens it will work.

Answer 3

Just return tokens

    class Lexer {
      // constructor and another method here
      make_tokens() {
        var tokens = [];
        // checking character values here
        console.log(tokens); 
        // Outputs [ Token { type: 'PLUS', value: undefined }, Token { type: 'PLUS', value: undefined } ] if I enter ++
        return tokens; // returns tokens
      }
      // make_numbers() goes here
    }
    
    const lexer = new Lexer();
    
    console.log(lexer.make_tokens());

Answer 4

Firstly, why you want to return null with other variable but if so then just return an array consisting of 2 elements.

Here's is the code:

class Lexer {
  // constructor and another method here
  make_tokens() {
    var tokens = [];
    // checking character values here
    console.log(tokens); 
    // Outputs [ Token { type: 'PLUS', value: undefined }, Token { type: 'PLUS', value: undefined } ] if I enter ++
    return [tokens, null] ; // returns undefined, null
  }
  // make_numbers() goes here
}

But it can be the other way. For eg You want to return null if tokens are not present.

class Lexer {
  // constructor and another method here
  make_tokens() {
    var tokens = [];
    // checking character values here
    console.log(tokens); 
    // Outputs [ Token { type: 'PLUS', value: undefined }, Token { type: 'PLUS', value: undefined } ] if I enter ++
    return tokens | null; // returns undefined, null
  }
  // make_numbers() goes here
}