Duplicate declaration in TypeScript

Question

Consider this code:

var a = [];

for (var i in a)
  /* nothing */;

var i : number;

i = 9;

If I compile this, the compiler complains about the last line, saying, "Cannot convert 'number' to 'string'".

Presumably the for..in loop created i as a string, but I would have expected the compiler to complain about var i : number rather than wait until i = 9 before it complained.

If I change var i : number to var i : number = 8, the compiler still doesn't complain until it reaches i = 9.

Is this a bug, or am I missing something?

Fenton · Accepted Answer · 2013-02-28 14:10:29Z

3

If you had originally declared the variable it would warn you where you predict. For example, if you placed this line at the top of your code:

var i: string;

It would warn you of the duplicate identifier on the line:

var i: number;

It doesn't detect the duplicate identifier when you declare i in the for statement, perhaps it should - so you end up with a different error, which is when you assign a value of the wrong type.

If you want the duplicate identifier warning, which is the first and foremost error here, your best bet is to log the bug on http://typescript.codeplex.com/

answered Feb 28, 2013 at 14:10

Fenton

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1 Comment

oz1cz Over a year ago

Thank you, Steve. I did as you suggested: typescript.codeplex.com/workitem/771

Alex Klock · Accepted Answer · 2013-02-28 14:11:45Z

0

It's not a bug, to understand what's going on you have to understand how JavaScript behaves, especially around the area regarding hoisting. Essentially, the code you have above is the same as:

var a = [],
    i;

for (i in a)
  /* nothing */;

i = 9;

Since i is first encountered in your for statement, its type is becoming a string. for (key: string in someVar) { }.

Here's an article that describes hoisting and scoping in JavaScript: http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting

answered Feb 28, 2013 at 14:11

Alex Klock

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4 Comments

oz1cz Over a year ago

But that doesn't explain why the compiler doesn't complain about var i : number = 8.

Alex Klock Over a year ago

The compiler complains for me when I do var i: number = 8;, at least in version 0.8.2.

oz1cz Over a year ago

I'm using 0.8.3, and it doesn't complain.

Jude Fisher Over a year ago

Must be a new bug in 0.8.3. It complains for me in 0.8.2, but not in typescriptlang.org/Playground Wierdly, in 0.8.3 you can trace typeof(i) and it will tell you 'number', but if you then try i * 10 you get the error described by @oz1cz in the OP.

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