您可以創建一個函數，該函數將調用一個參數，getSum但本身提供第一個參數。
TestClass.prototype.getSum = function() { //<–– normal function
return getSum.call( this, 1 );
// ^^^^ ^^^^ ^
// || || |
// forward this –++––––++ +–––––– pass a value for the first parameter
}
這將為您提供以下信息
function getSum (firstValue) { return firstValue + this.secondValue }
class TestClass {}
TestClass.prototype.getSum = function() {
return getSum.call(this, 1);
}
const obj = new TestClass()
obj.secondValue = 2
console.log(obj.getSum()) // 3
通常，將值綁定到函數中的參數的過程稱為部分應用程序。例如，如果一個函數需要三個參數，那么您一開始只能設置兩個，然后再設置最后一個。整個過程可以通過創建一個函數來處理這個抽象出來：
function partiallyApply(fn, ...params) {
return function(...moreParams) {
return fn.call(this, ...params, ...moreParams);
}
}
function takes4Parameters (a, b, c, d) {
return a + b + c + d;
}
const takes2Parameters = partiallyApply(takes4Parameters, 1, 2); // 1 + 2 + c + d
console.log("1 + 2 + 11 + 12 =", takes2Parameters(11, 12));
const takes1Parameter = partiallyApply(takes2Parameters, 3); // 1 + 2 + 3 + d
console.log("1 + 2 + 3 + 5 =", takes1Parameter(5));
const takesNoParameter = partiallyApply(takes1Parameter, 6); // 1 + 2 + 3 + 6
console.log("1 + 2 + 3 + 6 =", takesNoParameter());
使用那個高階函數，我們可以更容易地推導getSum出TestClass
function getSum (firstValue) { return firstValue + this.secondValue }
function partiallyApply(fn, ...params) {
return function (...moreParams) {
return fn.call(this, ...params, ...moreParams)
}
}
class TestClass {}
TestClass.prototype.getSum = partiallyApply(getSum, 1);
//example of adding other partially applied methods:
TestClass.prototype.getSum2 = partiallyApply(getSum, 2);
TestClass.prototype.getSum3 = partiallyApply(getSum, 3);
TestClass.prototype.getSum4 = partiallyApply(getSum, 4);
const obj = new TestClass()
obj.secondValue = 2
console.log(obj.getSum()); // 3
console.log(obj.getSum2()); // 4
console.log(obj.getSum3()); // 5
console.log(obj.getSum4()); // 6
