I'm curious about the meaning behind this phrase: "constructor(...[configuration])?"

Question

I'm curious about the meaning behind this phrase: "constructor(...[configuration])?"

I'm currently working with the CognitoIdentityClient class in the aws-sdk-js-v3 library, and I have come across a constructor that has me puzzled:

export class CognitoIdentityClient extends __Client<
  __HttpHandlerOptions,
  ServiceInputTypes,
  ServiceOutputTypes,
  CognitoIdentityClientResolvedConfig
> {
  constructor(...[configuration]: __CheckOptionalClientConfig<CognitoIdentityClientConfig>) {
    const _config_0 = __getRuntimeConfig(configuration || {});
    [...]
    }
}

What is the significance of the three dots and the brackets in this constructor? I tried calling it as follows, but encountered unexpected behavior:

new CognitoIdentityClient([myConfiguration]);

I attempted to create my own version, but I'm struggling to make it function properly:

test(...[myArgument]: string): void {
}
const myString: string = 'abc';
test(myString); //  Argument of type '[string]' is not assignable to parameter of type 'string'.
test([myString]); // Argument of type '[string[]]' is not assignable to parameter of type 'string'
test(...myString); // Argument of type 'string[]' is not assignable to parameter of type 'string'

javascript typescript

Answer 1

Answer №1

Similar to this pattern:

class SampleClass {
  constructor(...[param]: string[]) {
    console.log(param);
  }
}

Now it's evident that it is equivalent to:

class SampleClass {
  constructor(...args: string[]) {
    let [param] = args;
    console.log(param);
  }
}

An amalgamation of Rest Parameters and Destructuring Assignment.

Answer 2

Similar to this pattern:

class SampleClass {
  constructor(...[param]: string[]) {
    console.log(param);
  }
}

Now it's evident that it is equivalent to:

class SampleClass {
  constructor(...args: string[]) {
    let [param] = args;
    console.log(param);
  }
}

An amalgamation of Rest Parameters and Destructuring Assignment.

I'm curious about the meaning behind this phrase: "constructor(...[configuration])?"

Answer №1

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