类型错误：class.function 不是函数。 (在'classname.function(param)'中)

Question

I'm not an expert to JavaScript, but following the logic of languages like Java and others a the following should work, but does not:

//* @flow */

class Assets {
  parts: Array<PartsItem>;

  constructor() {
    this.parts = [];
  }

  merge(other: Assets) {
    this.parts = this.parts.concat(other.parts);
  }
}

class PartsItem {
  name: string;
}

function addNewAssets(assets: Assets, newAssets: Assets) {
  // the out log verifies that both assets are having all expected values and expected structure
  console.log("adding ", newAssets);
  console.log("to ", assets);
  assets.merge(newAssets);
  return assets;
}

function run_example() {
  let item1: PartsItem = new PartsItem();
  item1.name = "part1";
  let item2: PartsItem = new PartsItem();
  item2.name = "part2";

  let assets = new Assets();
  let otherAssets = new Assets();

  assets.parts.push(item1);
  otherAssets.parts.push(item1);

  let mergedAssets = addNewAssets(assets, otherAssets);

  console.log(JSON.stringify(mergedAssets));
}

run_example();

when calling addNewAssets I would expect to get merged assets as the result, but the result is:

[12:06:55] W | ReactNativeJS ▶︎ Possible Unhandled Promise Rejection (id: 0):
                             │ TypeError: assets.merge is not a function. (In 'assets.merge(newAssets)', 'assets.merge' is undefined)

Does it have anything to do with JS "unpredictable" this ?

Answer 1

Even when some very smart folks has downgraded that question, it is showcasing a pitfall with JS classes and Flow.

Since the assets has been used after being read back from JSON some class information has get lost, even tho the fields has been typed, Flow is not Java and not hard typed.

I'll need to try this: https://github.com/STRML/json-to-flow or https://www.npmjs.com/package/flow-validator as described here: How can I get JSON.parse result covered by the Flow type checker?