Writing produces type intersection in TS.

TS-example-writing-produces-intersection.ts

// https://github.com/microsoft/TypeScript/issues/47295
// https://stackoverflow.com/questions/70569105/proxy-property-in-setter-has-never-type
// https://www.typescriptlang.org/play?ts=4.6.0-dev.20220103#code/MYewdgzgLgBAtgTwPICMBWMC8MDeBYAKBhjAFc4AuGAWgEYAaQ4gQwCdWqBtBmAJnpgBmALqMiMaBxgByaWIC+AbkKFQkWAAdWIAB4IsJAKYB3GAAVtegBSJUaAfnERDUK1DYBzFwK0gNVAGtDBBAAMxgoBA1DMPhkdAEAN2YAG1JDAEpcJmIYAHoAKhJwQxhYqAALQ2cYYxBWAIgYAryc4jy8mAA1Q1YAOhhaNpgAS3CrXw1RsAjPFyz3Vi8oTknhA2S0w2Vxds6e-r5hxeXV7Q117E30ndz8-d6BwWO5lcmYZiagkPDI6NjbOhLjBrtthh1uo8YAAWYYQYwjKDACowCbnLKOO7EYCfUrSMhwaQUYbY3EySREkm5E4uM5+YGg25Y4goViGZgBJnEeQqXb5IrGCojFKlSrVUp1BpNFrgh6HACscIRSJRaL8GKpOJq+PIlL5dxpb3ODNSNypLLZHK5uS1eIpxP11NedIuG1NYMdFvZnOGPL5EIOAwAbErEcjUZMNY7bTICXrmcRDS6TVtrXdWd60zAACaGULMUgpKAOhOzJa0tZu1PmmAZq2+3l3NlQUisGZQVhm8TyBQZHaqcAQEAivopEAeKzSXx6GBUOQwacIPuEIA

const myObj = {
  num: -1,
  arr: [1, 2, 3],
  str: '',
};

const proxy = new Proxy(myObj, {
  set(target, prop: keyof typeof myObj, value) {
    /* none of these works */
    // Ver. 1
    if (prop in target) target[prop] = value;
    // Ver. 2
    target[prop] = value;
    // Ver. 3
    target[prop as keyof typeof myObj] = value;
    // Ver. 4
    switch (prop) {
      case 'num':
      case 'str':
        target[prop] = value;
        break;
    }

    /* while these works */
    // Ver. 5
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      case 'str':
        target[prop] = value;
        break;
    }
    // Ver. 6
    switch (prop) {
      case 'num':
        target[prop] = value;
        break;
      default:
        target[prop] = value;
        break;
    }

    return true;
  },
});

console.log('proxy : ', proxy);