javascript: two ways to use concat function

Question

below are two different ways to use concat. nameage and nameage2 produce somewhat different results, which also seem to be described differently in different browsers.

var name = ["david","ibrahim","lagan"];
var age = [23, 34, 24];

var nameage = name.concat(age);
var nameage2 = [name].concat(age);

for example, in chrome: console.log(nameage); produces "david,ibrahim,lagan23,34,24", whereas console.log(nameage2); produces "["david,ibrahim,lagan", 23, 34, 24]"

Can someone explain what the difference is between these two approaches, and how it might impact use of concat? (Is one way more 'correct'?)

Pointy · Accepted Answer · 2013-12-01 17:57:42Z

5

In the second one, you're putting the "name" array inside an array, as its first element, and then concatenating the "age" array onto that.

So:

[name]

gives you

[ [ "david", "ibrahim", "lagan" ] ]

It's an array inside of an array. When you concatenate onto that, you get

[ [ "david", "ibrahim", "lagan" ], 23, 34, 24 ]

It's not correct to think of the two samples you posted as two "approaches" — they're two quite different things. Which is "correct"? It depends on what result you want.

answered Dec 1, 2013 at 17:57

Pointy

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6 Comments

drenl Over a year ago

that makes sense; also it's consistent with the way Firefox displays the results. Do you have any idea why, in Chrome, the comma is omitted between "lagan" and "23"? It seems like it should be 'all one array', thus including the comma.

Pointy Over a year ago

Hmm .. Chrome isn't doing that for me when I console.log() those values. It shows the commas correctly.

drenl Over a year ago

for the first version (nameage)? interesting. it pulls the comma for me using both console.log() and document.write().

Pointy Over a year ago

@GregoryTippett wow Chrome is indeed doing some really weird stuff. I'm confused.

Pointy Over a year ago

@GregoryTippett ah - I think it has to do with using the variable name "name" in the global context. Try using "theName" instead of just "name". (The identifier "name" in the global context is special, as it has to do with the window's internal name? or something like that? I'm not sure.)

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