Member Avatar for itisnot_me

Ok so I am trying to create a loop that creates a dynamic variable but I cant find out how to do this. this is what I have so far

var map = { 
    'flammable': 'inflammable', 
    'duh': 'no duh' 
}; 

$.each(map, function(key, value) {  
   var key = value;
});

alert(duh);

But every time I run this is states that duh is not defined. What am I doing wrong?

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Short answer: you cannot turn a string into a JavaScript identifier at runtime with var. The var statement requires a literal identifier at parse time, and any var inside your jQuery callback is local to that function, so alert(duh) outside the callback still fails. See the MDN var reference for the syntax and scoping rules. MDN: var. (developer.mozilla.org)

About eval: and were on the right track that string-evaluation can create names, but eval parses and runs arbitrary code (security, debugging and performance problems) and its scoping behavior can be tricky (direct vs indirect eval). Avoid eval unless you have a very good reason. Read MDN’s guidance on eval() before using it. MDN: eval(). (developer.mozilla.org)

Better patterns (safer and simpler):

  • Keep your data in an object or an ES6 Map and access with map[key] — that is exactly what your original map is for, and it avoids all scope/parse-time problems.
  • If you truly need globals at runtime, set properties on the global object explicitly (in browsers window, or use the standard globalThis for portability): assign globalThis[key] = value. Note that modules and let/const behave differently and do not automatically become global properties. Prefer globalThis for cross-environment code. MDN: globalThis · MDN: Window. (developer.mozilla.org)

Practical takeaway: follow ’s diagnosis (there is no duh declared) and ’s scope warning, but avoid eval as suggested. The clean, maintainable solution is to use a container object (or Map) for dynamic keys and only use the global object explicitly when you really need a runtime-global binding.

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All 7 Replies

Member Avatar for azareth

it's because you didn't declare a var duh;that's why .. and you should assign a value to it .

Member Avatar for itisnot_me

Yes i know that but im trying to create a dynamic var. Where var key should create the duh var and the value will be no duh.
ex: var key = value; == var duh = 'no duh'

Member Avatar for |-|x

I believe you can do it like this:

eval("var " + key + " = " + value);
Member Avatar for itisnot_me

@hearth: That seems to work only with the first entry. Am I creating the array correctly?

Member Avatar for |-|x

the array looks ok to me.

eval("var " + key + " = " + value);

it might be to do with the eval'd code not having a semicolon, but i'm really just guessing at this point.

eval("var " + key + " = " + value + ";");

Maybe its a restriction of jQuery or the for-each loop... sorry, I'm not particularly a jQuery expert. :(

Member Avatar for somedude3488

You are running into scope issues. Variables created in a anonymous function are not available outside the function unless they are defined first outside the function (which defeats the purpose of what you are trying to do).

Try this maybe:

for( key in map ) {
    eval('var ' + key + ' = \'' + map[key] + '\'');
}

Its untested, but should give you an idea. Its best to avoid using eval if possible though.

Edited by somedude3488
commented: King +0
Member Avatar for itisnot_me

@kkeith29: Thanks sooooooo much that is what i was looking for. I am still new to javascript but i can def do php. you would think things would be more side by side.

What im trying to accomplish is sending through a array and break it apart in what i need. Basically im creating my own save/add function for my software.

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