JavaScript, async/await and promises

Question

I have a problem with async/await and some Promises.

I have this code. It starts here:

let valid = await LoadRouter.load(body); console.log(valid);//showing me Pending Promises

The function is:

loadGeneratingUnits(data){ let newUGArray = []; try { const result = data.map(async (itemGU, index) => { const checGU = await this.checkDataGu(itemGU.nombre); if(!checGU){ let newUG = { generating_unit_name: itemGU.nombre, description: (!itemGU.descripcion) ? null : itemGU.descripcion, it_generating_unit_id: (!itemGU.it_unidad_generadora) ? 0 : itemGU.it_unidad_generadora } newUGArray.push(newUG); } }) return result; } catch (error) { throw new Error(error.message) } }

This one is where I have the problems

async checkDataGu(guName = null){ if(guName){ return await generatingUnitModel.findOne({ attributes: [ 'id', 'generating_unit_name', ], where: { generating_unit_name: guName } }) }

}

Any comment about the use of async/await on this code?

Try making loadGeneratingUnits async, wrap the data.map in a Promise.all and await its result — cbr
– cbr, Commented Jan 6, 2020 at 21:21
The .map is now returning an array of promises since the mapper function returns promises (it's async!) — cbr
– cbr, Commented Jan 6, 2020 at 21:21
It would also appear that the function does not returm newUGArray. Should it? — cbr
– cbr, Commented Jan 6, 2020 at 21:25

Klaycon · Accepted Answer · 2020-01-06 21:33:45Z

By making the callback to data.map() async, data.map() is now transforming the data into an array of Promises, because the return value of an async function is always a Promise. await only will wait for a Promise to resolve, not an array of them. You should use Promise.all for that:

 const result = Promise.all(data.map(async (itemGU, index) => { const checGU = await this.checkDataGu(itemGU.nombre); if(!checGU){ let newUG = { generating_unit_name: itemGU.nombre, description: (!itemGU.descripcion) ? null : itemGU.descripcion, it_generating_unit_id: (!itemGU.it_unidad_generadora) ? 0 : itemGU.it_unidad_generadora } newUGArray.push(newUG); } }))

Now result is one Promise that will resolve with an array of the values each inner Promise resolved with. Ultimately this means your upper let valid = await LoadRouter.load(body); should resolve with the array you expect.

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