Map function returning array with index instead of string value

Question

Observing strange behavior in my map function:

let tpl = ({x}, y = `This is ${x}`) => y;
    
console.log( tpl({x:5}) ); //correctly returns 'This is 5'

console.log( [{x:1}, {x:9}].map(tpl) ); //expecting array of strings, instead get [0, 1]

It is possible to 'fix' this by omitting default variable y, and returning the template string directly from tpl. However, I would prefer to do the initialization this way and can't understand why it wouldn't still work.

Seems like a strange anomaly to me, does someone have insight on something I am missing?

Why exactly do you prefer to initialize it with y when you can just return the template string directly like you said? Is it because you are sometimes passing in an optional template string to override the default one? — Josh Crozier
– Josh Crozier, Commented Feb 1, 2017 at 4:32
This is a sort of contrived example of what I want to do (y for example, could be the result of a function call), but it's mostly about aesthetics; I like the concept of one character returns — Conqueror
– Conqueror, Commented Feb 1, 2017 at 4:49

Tom Coughlin · Accepted Answer · 2017-02-01 04:20:41Z

3

Your tpl function is provided (value, index, array) by JavaScript's map Array method. You've given y a default value of This is ${x}. But when you pass tpl as the callback to map, y is not undefined so the default value is not used. map is passing the index to your y parameter.

let tpl = ({x}, y = `This is ${x}`) => y
const arr = [ {x:1}, {x:9} ];
arr.map(tpl) // [0, 1]
arr.map((value, index, array) => tpl(value)) // [ 'This is 1', 'This is 9' ]

Array.prototype.map()

Default parameters

answered Feb 1, 2017 at 4:20

Tom Coughlin

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2 Comments

Conqueror Over a year ago

Thank you for this insight - this clarified my confusion.

Tom Coughlin Over a year ago

Awesome, no problem @Conqueror

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