Demonstration of why not to use the with statement.

README.md

Simulating a with statement to demonstrate the dangers.

https://jsfiddle.net/2eqsoqoc/

how_with_works.js

/**
 * how_with_works.js                                                            2016.12.22 HeilTec
 * 
 * To demonstrate how the 'with' statement works and why it can hide mistakes
 * this code outlines some of the possible outcomes of this construction:
 *   --------------------------
 *   | with (unknownObject) { |
 *   |   result = value       |
 *   | }                      |
 *   --------------------------
 * In a nodejs process the global object is named 'global' and in a browser it is called 'window'
 * 
 * to test the 4 outcomes that doesn't fail uncomment any one of the following:
 */

// unknownObject = { other:0 };            value = 111;
// unknownObject = { result: undefined };  value = 222;
// unknownObject = {                       value:  333 };
// unknownObject = { result: undefined,    value:  444 };

/**
 * Provoke bad errors by uncommenting one of these.
 */

// var unknownObject;
// var unknownObject=1;

simulate = 1;
if (simulate) {
    try {
        if (!unknownObject) { // falsy fails if not defined 
            console.error('The unknownObject was null or undefined');
        } else {
            if (Object.keys(unknownObject).indexOf('value') === -1) {
                if (Object.keys(unknownObject).indexOf('result') === -1) {
                    try {  
                        result = value; // global.result = global.value
                    } catch(e){
                        console.error('The value was never allocated', e.message);
                    }
                } else {
                    unknownObject.result = value; // global.value
                }
            } else {
                if (Object.keys(unknownObject).indexOf('result') === -1) {
                    result = unknownObject.value; // global.result
                } else {
                    unknownObject.result = unknownObject.value; // Both in the object
                }
            }
        }
    }
    catch(e) { 
        console.error('The unknownObject was never allocated\r\n', e.message); 
    }
} else {
    try {
        with (unknownObject) { // [jshint] Don't use 'with'. (W085)
            result = value;
        }                     
    } catch(e) { 
        console.log('Runtime: ', e.message); 
    }
}

/**
 * Some mistakes are caught by runtime. 
 * Reference errors will occur when the object is not defined 
 * or the global.value is used and not defined.
 * 
 * Notice that it is only through try/catch that 'undefined' can be differentiated from not defined. 
 */
try {
    console.log('unknownObject.result: ', unknownObject.result);
} catch(e) { 
    console.log('Result: ', e.message);
}
try {
    console.log('global.result: ', result);
} catch(e) { 
    console.log('Result: ', e.message);
}