具有高阶函数的字符串方法

Question

I ran into a weird thing while trying to use String methods with higher-order functions. This will throw an error:

['a', 'b'].some('boo'.includes)

I have to wrap the predicate in another function to make it work. But isn't 'boo'.includes already a function?

This works with plain functions:

const boo = {
    includes: () => true
};

['a', 'b'].some(boo.includes)

Is there some special property of String methods that prevents them from being composed like this?

Answer 1

"boo".includes is nothing else than String.prototype.includes . Calling it on the string "boo" however sets this to "boo", which gives the proper context to the function. Eg "boo".includes("b") is the same as String.prototype.includes.call("boo", "b") .

Passing it as an argument, eg ['a', 'b'].some('boo'.includes) , is the same as ['a', 'b'].some(String.prototype.includes) , which then lacks the proper this as context.

You can of course use eg bind : ['a', 'b'].some(String.prototype.includes.bind("boo")) , or use the optional second parameter thisArg for some : ['a', 'b'].some(String.prototype.includes, "boo") . This will get rid of the error. However, you will then notice, that some passes not only the element, but the index as second parameter, and the array itself as third. This is a problem, as includes also takes an optional second parameter for the start position. This causes likely unwanted behavior, as eg the array element "b" is at index 1, and "boo".includes("b", 1) === false .

All in all, you need a function which is so different to String.prototype.includes , that it's just easier to wrap it in a new function that actually does what you want: ['a', 'b'].some(e => "boo".includes(e)) , as you already noticed.

Answer 2

Scott Marcus's comment is incorrect. It's completely normal in functional programming to pass around functions without their required arguments. Here's the actual problem:

The callback you pass to some is called with three arguments: currentValue , index , and array , and its this is set to undefined unless you pass a second parameter to some . The method String.prototype.includes takes two arguments: search and start , and its this is used as the string to be searched. As a result of this interaction, includes gets called without a string to search, so it gives an error.

Even if you fix the above via bind or by passing a second parameter to some , this won't entirely fix the problem though. The other half of the problem is the second argument. some considers it to be index but includes considers it to be start . As a result, the b won't be searched for until the second character of the string, so even though the error will be gone, the test will still return false.

In this case, a wrapper function is unavoidable to get the desired behavior.