0

Here's my object:

var myObj = 
{
    nestedObj: {
        "My key has spaces": "You want my value"
    }
}

I don't know the content of myObj when it reaches me. I have a string variable representing the key for the value I need. Looks like this:

var theKey = "nestedObj['My key has spaces']";

How do I use this string variable to get the value from the object?

The obvious guess, which does not work:

myObj[theKey]; // returns undefined

Edit: I'm hoping for a solution that's more flexible than this particular example. I.e., I may have to go another level deep, and dot notation could be thrown in the mix. I'm using angular.js, and was thinking there must be something that does this already.

CDelaney
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  • Adapt [Accessing nested JavaScript objects with string key](http://stackoverflow.com/a/6491621/218196) to work with string literals inside the bracket notation (it already supports bracket notation). – Felix Kling Mar 27 '15 at 21:10
  • 1
    possible duplicate of [Accessing nested JavaScript objects with string key](http://stackoverflow.com/questions/6491463/accessing-nested-javascript-objects-with-string-key) – nicael Mar 27 '15 at 21:12

4 Answers4

0
  1. Split the string taking [' as a delimiter
  2. The first part would be nestedObj - that's the name of the first nested object
  3. The second part is My key has spaces']; we'll remove '] from the end by replacing it with empty string; so now we have the property of the nested object - exactly what we need
var theKey = "nestedObj['My key has spaces']";

myObj[theKey.split("['")[0]][theKey.split("['")[1].replace("']","")];

Demo:

var myObj = 
{
    nestedObj: {
        "My key has spaces": "You want my value"
    }
}

var theKey = "nestedObj['My key has spaces']";

var result = myObj[theKey.split("['")[0]][theKey.split("['")[1].replace("']","")];

alert(result);
nicael
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  • Thanks, I'm looking for a little more flexible solution than this though. I edited my post to include that I'm using angular.js, and that it could be more than two levels deep and could include dot notation. – CDelaney Mar 27 '15 at 21:10
0

Try following:

eval('myObj.' + theKey);
yas
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  • This works, but isn't eval supposed to be avoided at all costs? – CDelaney Mar 27 '15 at 21:15
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    @CDelaney: No. `eval` should be avoided if you are working with values from an unknown (i.e. not trustworthy) source or if there is a (relatively) simple solution without it. – Felix Kling Mar 27 '15 at 21:17
  • @FelixKling Ah, that makes sense. Thanks, I'll probably use this as a solution then. – CDelaney Mar 27 '15 at 21:19
0

EDIT: Improved answer for updated question :)

Link to JsFiddle

Use regular expressions:

var test1 = "nestedObj['My key has spaces']";
var test2 = "abc [\"def ghi\" ].jkl[123][ 'mn o']";

function getKeys(str) {
    // Regular expressions for the first key in the str
    var reg = [
        /^\s*\[\s*(\d+)\s*\]/,         // Case 1: [ someIntegerKey ]
        /^\s*\[\s*\'([^\']+)\'\s*\]/,  // Case 2: [ 'someKey' ]
        /^\s*\[\s*\"([^\']+)\"\s*\]/,  // Case 3: [ "someKey" ]
        /^\s*\.\s*([^\[\.]+)/          // Case 4: . someKey
    ];

    var keys = [];
    var tail = "." + str;
    var success = true;
    while (success) {
        var result = 
            tail.match(reg[0]) || 
            tail.match(reg[1]) ||
            tail.match(reg[2]) || 
            tail.match(reg[3]);

        if (result) {
            keys.push(result[1]);
            tail = tail.slice(result[0].length);
        } else {
            success = false;
        }
    }
    return keys;
}

console.log(getKeys(test1));
console.log(getKeys(test2));
Radek
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0

It's most likely that you could get away with a simple array of strings, in which case the code to do this is much simpler (originally posted by me in another answer):

function get(obj, property) {
  var current = obj;
  for (var i = 0, l = property.length; i < l; ++i) {
    if (Object(current) === current) current = current[property[i]];
    else {
      current = undefined;
      break;
    }
  }
  return current;
}

Example:

get(obj, ['nestedObj', 'My key has spaces']); // "You want my value"

This just returns undefined if the property could not be found, but if you'd like the typical JavaScript behaviour of throwing an error when attempting to access a property on undefined or null, it's simple enough to edit the code to change that.

However, if you're really looking to use the exact property string syntax, I wrote a parser for this task a long time ago, but it's rather crude. Basically it uses regular expressions to match segments of the string, returning them as an array. It then goes through the array to find nested values of an object passed in.

This parser supports full escape sequences, strings, plain numbers and dot property access. The only thing it doesn't support is referencing actual variables and using invalid identifiers at the beginning (working on that).

Example:

evaluateProperty({ test: { 'space space': 1 } }, 'test["space space"]'); // 1

Code:

function parsePropertyString(str) {
  function parseStr(str, quote) {
    var stack = [];
    var arr = str.split('');

    var escapeChars = {
      b: '\b',
      t: '\t',
      n: '\n',
      v: '\x0B',
      f: '\f',
      r: '\r'
    };
    var SingleEscapeCharacter = /^[bfnrtv]/;
    var HexEscapeSequence = /^x([\da-fA-F]{2})/;
    var UnicodeEscapeSequence = /^u([\da-fA-F]{4})/;
    var NullEscapeSequence = /^0(?!\d)/;
    var OctalEscapeSequence = /^(?:0[0-7]|[1-3][0-7]?|[4-7])[0-7]?/;
    var LineTerminatorSequence = /^\r\n?|[\n\u2028\u2029]/;
    var InvalidEscapeSequence = /^[\dxu]/;

    while (arr.length) {
      var character = arr[0];
      if (character === quote) break;
      if (LineTerminatorSequence.test(character)) throw new SyntaxError('Unexpected token ILLEGAL');
      if (character === '\\') {
        arr.shift();
        character = arr[0];
        var currentStr = arr.join('');
        if (SingleEscapeCharacter.test(character)) {
          stack.push(escapeChars[character]);
          arr.shift();
          continue;
        }

        var match;
        if (match = currentStr.match(HexEscapeSequence) || currentStr.match(UnicodeEscapeSequence)) {
          if (match[1]) stack.push(String.fromCharCode(parseInt(match[1], 16)));
          arr.splice(0, match[0].length);
          continue;
        }
        if (match = currentStr.match(OctalEscapeSequence)) {
          stack.push(String.fromCharCode(parseInt(match[1], 8)))
          arr.splice(0, match[0].length);
          continue;
        }
        if (match = currentStr.match(LineTerminatorSequence)) {
          arr.splice(0, match[0].length);
          continue;
        }
        if (NullEscapeSequence.test(currentStr)) {
          stack.push('\0');
          arr.shift();
          continue;
        }
        if (InvalidEscapeSequence.test(currentStr)) throw new SyntaxError('Unexpected token ILLEGAL');
        // non-escape characters will be treated as normal (backslash will be ignored)
      }

      stack.push(arr.shift());
    }
    return stack.join('');
  }

  var DotPropertyName = 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  var SingleStrBracket = /^\[\s*'((?:[^'\\\r\n\u2028\u2029]|\\(?:x[\da-fA-F]{2}|u[\da-fA-F]{4}|0(?!\d)|[^xu\d\r\n\u2028\u2029])|(?:\r\n|[\r\n\u2028\u2029]))*?)(')\s*\]/;
  var DoubleStrBracket = /^\[\s*"((?:[^"\\\r\n\u2028\u2029]|\\(?:x[\da-fA-F]{2}|u[\da-fA-F]{4}|0(?!\d)|[^xu\d\r\n\u2028\u2029])|(?:\r\n|[\r\n\u2028\u2029]))*?)(")\s*\]/;
  var NumberBracket = /^\[\s*([+-]?\s*(?:(?!\.?(?:[eE][+-]?\d+?)?$)(?:[1-9]\d*?|0)?(?:\.\d*?)?(?:[eE][+-]?\d+?)?|0[xX][\da-fA-F]+?))\s*\]/;
  var NumberLiteral = /^([+-]?\s*(?:(?!\.?(?:[eE][+-]?\d+?)?$)(?:[1-9]\d*?|0)?(?:\.\d*?)?(?:[eE][+-]?\d+?)?|0[xX][\da-fA-F]+?))/;
  var StrWhiteSpace = /^\s+/;

  var properties = [];

  // reusing DotPropertyName as if it was an IdentifierName
  var part = ('.' + str).match(DotPropertyName);
  if (!part) throw new SyntaxError('Unexpected token ILLEGAL');
  var arr = str.split('').slice(part[0].length - 1); // -1 to take into account the dot
  properties.push(part[1]);

  while (arr.length) {
    var character = arr[0];
    var nextChar = arr[1];
    var partsStr = arr.join('');

    // convert the string to number and back (representation may be different, deliberate assignment)
    if (part = partsStr.match(NumberBracket)) {
      arr.splice(0, part[0].length);
      properties.push(+part[1] + '');
    }
    // just to stop it throwing error (deliberate assignment)
    else if (part = partsStr.match(StrWhiteSpace) || partsStr.match(DotPropertyName)) {
      arr.splice(0, part[0].length);
      if (part[1]) properties.push(part[1]);
    }
    else if (part = partsStr.match(SingleStrBracket) || partsStr.match(DoubleStrBracket)) {
      arr.splice(0, part[0].length);
      properties.push(parseStr(part[1], part[2]));
    }
    else throw new SyntaxError('Unexpected token ILLEGAL');
  }
  return properties;
}

function evaluateProperty(obj, str) {
  var propertyChain = parsePropertyString(str);
  for (var i = 0; i < propertyChain.length; ++i) obj = obj[propertyChain[i]];
  return obj;
}

// avoid repetition of regex
function createStrRegex(type) {
  return new RegExp(
    '^' + type + '(?:[^' + type +
    '\\\\\\r\\n\\u2028\\u2029]|\\\\(?:[^xu\\d\\r\\n\\u2028\\u2029]|0(?!\\d)|x[\\da-fA-F]{2}|u[\\da-fA-F]{4})|(?:\\r\\n|[\\r\\n\\u2028\\u2029]))*?' +
    type + '$'
  );
}

var singleQuoteStr = createStrRegex("'");
var doubleQuoteStr = createStrRegex('"');
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Qantas 94 Heavy
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  • This is the type of thing I was looking for, but I'm really surprised it isn't already in a library like jquery. – CDelaney Mar 28 '15 at 22:11
  • @CDelaney: if you don't need the exact property access format, this code can be made much simpler by using an array like `['nestedObj', 'My key has spaces']`, which isn't really special enough to include in a library like jQuery. – Qantas 94 Heavy Mar 29 '15 at 01:27
  • I don't have control over the format of the key or the object it's accessing, so I don't think this is an option. – CDelaney Mar 30 '15 at 14:26
  • @CDelaney: in that case, I don't think you'd find this in any library. You're probably better off just using the solution here. – Qantas 94 Heavy Mar 31 '15 at 05:29