Accumulating Parameter Pattern in JavaScript #javascript #typescript

rec_params.ts

// mutating input and output
const rec = (inputs: Array<number>, outputs: Array<number> = []) => {
  if (!inputs.length) {
    return outputs;
  } else {
    const input = inputs.shift()!;
    outputs.push(input + 1);
    return rec(inputs, outputs);
  }
};
console.log(rec([1,2,3,4]));

// constant input and mutating output (this is probably the fastest)
const inputs = [1,2,3,4];
const rec2 = (inputsIndex: number, outputs: Array<number> = []) => {
  if (inputsIndex >= inputs.length) {
    return outputs;
  } else {
    const input = inputs[inputsIndex];
    outputs.push(input + 1);
    return rec2(inputsIndex + 1, outputs);
  }
};
console.log(rec2(0));

// immutable input and output
const rec3 = ([input, ...inputs]: Array<number>, outputs: Array<number> = []) => {
  if (input == undefined) {
    return outputs;
  } else {
    return rec3(inputs, outputs.concat(input + 1));
  }
};
console.log(rec3([1,2,3,4]));

// you can mix and match the above based on performance requirements

// this pattern can be used with callbacks
// which can be necessary to loop sequence asynchronous callbacks

const callback = (e, r) => {
  console.log(r);
};
const rec4 = ([input, ...inputs]: Array<number>, outputs: Array<number> = []) => {
  if (input == undefined) {
    callback(null, outputs);
  } else {
    setTimeout(() => {
      outputs.push(input + 1);
      rec4(inputs, outputs);
    });
  }
};
rec4([1,2,3,4]);