Docs: Partial Example Error from v3.5.1

[hookercookerman]

Partial Example: error from 3.5.1

interface NodeConfig {
 		appName: string;
 		port: number;
 }

 class NodeAppBuilder {
 		private configuration: NodeConfig = {
 				appName: 'NodeApp',
 				port: 3000
 		};

 		config(config: Partial<NodeConfig>) {
 				type NodeConfigKey = keyof NodeConfig;

 				for (const key of Object.keys(config) as NodeConfigKey[]) {
 						const updateValue = config[key];

 						if (updateValue === undefined) {
 								continue;
 						}

 						this.configuration[key] = updateValue;
 				}

 				return this;
 		}
 }

 // `Partial<NodeConfig>`` allows us to provide only a part of the
 // NodeConfig interface.
 new NodeAppBuilder().config({appName: 'ToDoApp'});

Type 'string | number' is not assignable to type 'string & number'.
  Type 'string' is not assignable to type 'string & number'.
    Type 'string' is not assignable to type 'number'.

playground example

https://www.typescriptlang.org/play/?ts=3.3.3#code/JYOwLgpgTgZghgYwgAgHIHsAmEDC6QzADmyA3gLABQyycADnanALYQBcyAzmFKEQNxUaddFDAcQAV2YAjaIMoBfKlQQAbOJ05osEAIIMAQpOBrsUMkOR1eANziRkCfISKSoD4Pg4ZseAsTIALyW1DS0DEysHADkvvoMMQA0VsKi4sgAzAAMuVaKClbOAUQAFMWuHAAKcGLAcGoAPPH+rgB8AJSh4chgAJ50KC0uxADSEH3ByADWE+gwOn4jAiphNDCiyOX43DMTyPPIAPIyAFYQCGAAdLN9nNslXZqLuMvjfQDaALpdFGvhxV2kjomAcEAAag1JCgQhViB9bl9Cv8aMAFqVgaDIJC1NDgkEQpIQNhCCAIJhfqkek58GBQNCFNTkMpKFSaGAABbAThXOFuDx0-AIiZfKaYsE46FUllUqAQMDuEC9LmcRnMqgsoA

I would submit pull request but my typescript journey is just beginning; nut shell either typescript bug or from 3.5.1 strictness on this is tighter. But for me both types are string | number| so 🤷‍♂

[sindresorhus]

// @krnik

[ghost]

Hi, I'll respond and try to fix the code on 7th Sept.

But, what's happening here is most likely caused by TypeScripts' 3.5 Fixes to unsound writes to indexed access types.

It's not really an access issue but assign issue. Since simply accessing the property by key works fine.
Any ideas why accessing and assigning gives different types?

  const x = this.configuration[key]; // string | number
  this.configuration[key] /* string & number */ = <value>; // TS 3.5
  this.configuration[key] /* never */ = <value>; // TS 3.6

---

Edit:
Related TS PR link

In a nut shell: indexed key assignment is more strict (sinze 3.5) and allows only writes that are certain to give correct result.