Resolve promises in object tree

Question

1. How do I resolve foo3

2. How can I resolve the object tree without async/await with only plain Promise (without depending on regenerator-runtime )

 const p = v => { return new Promise(resolve => setTimeout(resolve.bind(null, v), 2000) ) } const tree = { element: p('foo'), children: [ p({ element: 'bar', children: null }), p({ element: 'bar2', children: [ { element: p('foo3'), children: null } ] }) ] } const fullfill = async vtree => { if(vtree instanceof Promise) { await vtree } else if(Array.isArray(vtree)) { await Promise.all(vtree.map(fullfill)) } else if(typeof vtree !== 'object') { return vtree } else { let { element, children = [] } = vtree if(element instanceof Promise) { element = await element } if(children.length) { children = await Promise.all(children.map(fullfill)) } return { element, children } } return vtree } fullfill(tree).then(console.log)

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Answer 1

Problem with your code was your children value of tree object was processed by first if (vtree instanceof Promise) { . And their child never processed.

To properly process child I removed assigned assign awaited object back to vtree inside first if (Change 1) & add vtree = await fullfill(vtree) next to it (Change 2).

Use children && children.length so if children is null then it won't throw exception. (Change 3)

Test it below.

 const p = v => { return new Promise(resolve => setTimeout(resolve.bind(null, v), 2000) ) } const tree = { element: p('foo'), children: [ p({ element: 'bar', children: null }), p({ element: 'bar2', children: [{ element: p('foo3'), children: null }] }) ] } const fullfill = async vtree => { if (vtree instanceof Promise) { // Chanage 1 // assign awaited object back to vtree vtree = await vtree; // Chanage 2 // Call fullfill and pass vtree vtree = await fullfill(vtree); } else if (Array.isArray(vtree)) { await Promise.all(vtree.map(fullfill)) } else if (typeof vtree !== 'object') { return vtree } else { let { element, children = [] } = vtree if (element instanceof Promise) { element = await element } // Chanage 3 // use children && children.length so if children is null then it won't throw exception. if (children && children.length) { children = await Promise.all(children.map(fullfill)); } return { element, children } } return vtree } fullfill(tree).then(console.log)

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---

Edit without await . To remove await we need to return Promise . Please review first code snippet and then check equivalent version in second snippet which return Promise instead of await .

Test it below.

 const p = v => { return new Promise(resolve => setTimeout(resolve.bind(null, v), 2000) ) } const tree = { element: p('foo'), children: [ p({ element: 'bar', children: null }), p({ element: 'bar2', children: [{ element: p('foo3'), children: null }] }) ] } const fullfill = async vtree => { if (vtree instanceof Promise) { // Chanage 1 // assign awaited object back to vtree return vtree.then(r => fullfill(r)) //vtree = await vtree; //vtree = await fullfill(vtree); } // Chanage 2 // update else if condition to if here. else if (Array.isArray(vtree)) { return Promise.all(vtree.map(fullfill)) } else if (typeof vtree !== 'object') { return vtree } else { let { element, children = [] } = vtree if (element instanceof Promise) { // element = await element return element.then(e => { if (children && children.length) { return Promise.all(children.map(fullfill)).then(c => ({ element: e, children: c })); }; return { element: e, children: children }; }); } // Chanage 3 // use children && children.length so if children is null then it won't throw exception. else if (children && children.length) { return Promise.all(children.map(fullfill)).then(c => ({ element: element, children: c })); } } return vtree } fullfill(tree).then(console.log)

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Answer 2

There's hardly a reason to use async / await if all you're doing is to return the result anyway. You'd only use it for the tree node (element+children) case, but there you can easily do the transformation after the necessary Promise.all with then as well:

function treeAll(vtree) {
  if (typeof vtree !== 'object' || vtree == null) {
    return Promise.resolve(vtree);
  } else if (vtree instanceof Promise) {
    return vtree.then(treeAll);
  } else if (Array.isArray(vtree)) {
    return Promise.all(vtree.map(treeAll));
  } else {
    return Promise.all([
      vtree.element,
      treeAll(vtree.children)
    ]).then(([element, children]) => ({
      element,
      children
    }));
  }
}