45 lines
1.7 KiB
JavaScript
45 lines
1.7 KiB
JavaScript
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//#region src/function/partial.ts
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/**
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* Creates a function that invokes `func` with `partialArgs` prepended to the arguments it receives. This method is like `bind` except it does not alter the `this` binding.
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*
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* The partial.placeholder value, which defaults to a `symbol`, may be used as a placeholder for partially applied arguments.
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*
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* Note: This method doesn't set the `length` property of partially applied functions.
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*
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* @template F The type of the function to partially apply.
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* @param {F} func The function to partially apply arguments to.
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* @param {any[]} partialArgs The arguments to be partially applied.
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* @returns {(...args: any[]) => ReturnType<F>} Returns the new partially applied function.
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*
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* @example
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* function greet(greeting, name) {
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* return greeting + ' ' + name;
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* }
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*
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* const sayHelloTo = partial(greet, 'hello');
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* sayHelloTo('fred');
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* // => 'hello fred'
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*
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* // Partially applied with placeholders.
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* const greetFred = partial(greet, partial.placeholder, 'fred');
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* greetFred('hi');
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* // => 'hi fred'
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*/
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function partial(func, ...partialArgs) {
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return partialImpl(func, placeholderSymbol, ...partialArgs);
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}
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function partialImpl(func, placeholder, ...partialArgs) {
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const partialed = function(...providedArgs) {
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let providedArgsIndex = 0;
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const substitutedArgs = partialArgs.slice().map((arg) => arg === placeholder ? providedArgs[providedArgsIndex++] : arg);
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const remainingArgs = providedArgs.slice(providedArgsIndex);
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return func.apply(this, substitutedArgs.concat(remainingArgs));
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};
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if (func.prototype) partialed.prototype = Object.create(func.prototype);
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return partialed;
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}
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const placeholderSymbol = Symbol("partial.placeholder");
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partial.placeholder = placeholderSymbol;
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//#endregion
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export { partial, partialImpl };
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