How to replace `with` statement in strict mode

Question

This code works optimally and is easily comprehensible:

function evalInScope(js, contextAsScope) {
    //# Return the results of the in-line anonymous function we .call with the passed context
    return function() {
        with(this) {
            return eval(js);
        };
    }.call(contextAsScope);
}
evalInScope("a + b", {a: 1, b: 2}); // 3 obviously, but fails in strict mode!

However, the "smart" brains decided to remove the with statement without a proper replacement.

Question: how to make it work again in ES6, which is automatically in strict mode?

Answer 1

Don't use eval, create a new Function instead. It won't inherit lexical strict mode - and even better, it won't inherit all your function-scoped and module-scoped variables:

"use strict";

function evalInScope(js, contextAsScope) {
  return new Function(`with (this) { return (${js}); }`).call(contextAsScope);
}

console.log(evalInScope("a + b", { a: 1, b: 2 })); // 3

Also you don't get the weird "(last) statement result" return value that eval uses, but can either confine the js code to be an expression or include a return statement in the code itself.

---

Alternatively, if you don't actually need to use a with statement with all its intricacies but just want to make a dynamic set of constant variables available to the evaled code, just generate the code for those constants dynamically. This allows to eval the code in strict mode even:

"use strict";

function evalInScope(js, contextAsScope) {
  return new Function(
    `"use strict";
    const {${Object.keys(contextAsScope).join(', ')}} = this;
    return (${js});`
  ).call(contextAsScope);
}

console.log(evalInScope("a + b", { a: 1, b: 2 })); // 3

Or if the code doesn't use the this keyword itself, maybe also

"use strict";

function evalInScope(js, contextAsScope) {
  return new Function(
    '{' + Object.keys(contextAsScope).join(', ') + '}',
    `return (${js});`
  )(contextAsScope);
}

console.log(evalInScope("a + b", { a: 1, b: 2 })); // 3