How to set the value of this when evaluating a SES program?

[chetbox]

We're migrating from another Javascript-embedded language, JEXL, where we use this to refer to a particular constant object value.

e.g. Our user can use an expression such as this.value + 1 to increment a value.

How do I set the value of this in a compartment? Setting this on the global object doesn't seem to work:

const compartment = new Compartment({ __options__: true, globals: { this: { value: 1 }}});
console.log(compartment.evaluate('this.value')) // undefined

[gibson042]

I guess you can either prefix references to this in evaluated code:

const compartment = new Compartment({ __options__: true, globals: { this: { value: 1 }}});
console.log(compartment.evaluate('globalThis.this.value')) // 1

or wrap evaluated code in a function call whose receiver is the desired object

const compartment = new Compartment({ __options__: true, globals: { this: { value: 1 }}});
const wrapSourceForThis = expr => {
  const functionBody = `return(\n${expr}\n);`;
  Function(functionBody); // check validity
  return `Reflect.apply(function(){${functionBody}},globalThis.this,[])`;
};
console.log(compartment.evaluate(wrapSourceForThis('this.value'))) // 1

And in in either case, you'll want to decide on appropriate behavior for recursive evaluation.

[kriskowal]

The keyword this is not a lexical binding in JavaScript, so although you can see globalThis.this.value === 1 in the above compartment, in the top level scope of evaluate, this should be undefined at the top level in strict mode, but the SES shim can’t do that so we fall back to the expected behavior of sloppy mode, which is globalThis === this.

To achieve the effect that you are looking for, you can use a compartment’s unique Function constructor and bind this explicitly.

const c = new Compartment();
console.log(new c.globalThis.Function('return this.value').call({ value: 1 }));

[chetbox]

Ah thanks, this could be really helpful! Does running c.globalThis.Function('return this.value').call({ value: 1 }) create and run the function in the new sandboxed environment? Can I pass any JS object into .call()?

This could be a workaround for #2727 too if I can pass the object I want available as "globals" in.