---
robots: noindex, nofollow
tags: refactoring
---

# Split Variable

> 練習用的 [REPL](https://repl.it/@yaosiang/SplitVariable)

function distanceTravelled(scenario, time) {
    let result;
    let acc = scenario.primaryForce / scenario.mass; // acc 賦值一次
    let primaryTime = Math.min(time, scenario.delay);
    result = 0.5 * acc * primaryTime * primaryTime;
    let secondaryTime = time - scenario.delay;
    if (secondaryTime > 0) {
        let primaryVelocity = acc * scenario.delay;
        acc = (scenario.primaryForce + scenario.secondaryForce) / scenario.mass; // acc 賦值第二次
        result += primaryVelocity * secondaryTime + 0.5 * acc * secondaryTime * secondaryTime;
    }
    return result;
}

把一個變數變成多個變數，每個負責不同任務。

function distanceTravelled(scenario, time) {
    let result;
    const primaryAcceleration = scenario.primaryForce / scenario.mass;  // 先將 acc 改名字，並且用 const
    let primaryTime = Math.min(time, scenario.delay);
    result = 0.5 * primaryAcceleration * primaryTime * primaryTime;
    let secondaryTime = time - scenario.delay;
    if (secondaryTime > 0) {
        let primaryVelocity = primaryAcceleration * scenario.delay;
        let acc = (scenario.primaryForce + scenario.secondaryForce) / scenario.mass;
        result += primaryVelocity * secondaryTime + 0.5 * acc * secondaryTime * secondaryTime;
    }
    return result;
}

function distanceTravelled(scenario, time) {
    let result;
    const primaryAcceleration = scenario.primaryForce / scenario.mass;
    let primaryTime = Math.min(time, scenario.delay);
    result = 0.5 * primaryAcceleration * primaryTime * primaryTime;
    let secondaryTime = time - scenario.delay;
    if (secondaryTime > 0) {
        let primaryVelocity = primaryAcceleration * scenario.delay;
        const secondaryAcceleration = (scenario.primaryForce + scenario.secondaryForce) / scenario.mass;    // 將第二個 acc 改名字，也用 const
        result += primaryVelocity * secondaryTime +
            0.5 * secondaryAcceleration * secondaryTime * secondaryTime;
    }
    return result;
}

另外一種情況會發生在參數傳遞過程。

function discount(inputValue, quantity) {
    if (inputValue > 50) inputValue = inputValue - 2;
    if (quantity > 100) inputValue = inputValue - 1;
    return inputValue;
}

function discount(originalInputValue, quantity) {
    let inputValue = originalInputValue; // 先 clone 一份
    if (inputValue > 50) inputValue = inputValue - 2;
    if (quantity > 100) inputValue = inputValue - 1;
    return inputValue;
}

function discount(inputValue, quantity) {
    let result = inputValue; // 改名字
    if (inputValue > 50) result = result - 2; // 這裡仍用 inputValue，表明我是要跟 parameter 比對
    if (quantity > 100) result = result - 1;
    return result;
}