Javascript

Problem:

You want to run a NodeJS app using

npm start

but you only see this error message:

npm ERR! missing script: start

Solution:

You need to tell npm what to do when you run npm start explicitly by editing package.json.

First, identify the main file of your application. Most often it is called index.js, server.js or app.js. When you open package.json in an editor, you can also often find a line like

"main": "index.js",

In this example, index.js is the main file of the application.

Now we can edit package.json to add a start script.

In package.json, find the "scripts" section. If you have a default package.json, it will look like this:

"scripts": {
  "test": "echo \"Error: no test specified\" && exit 1"
},

Now add a comma to the end of the "test" line and add this line after it:

"start": "node index.js"

and replace index.js by the main file of your application (e.g. app.js, server.js, index.js etc).

The "scripts" section should now look like this:

"scripts": {
  "test": "echo \"Error: no test specified\" && exit 1",
  "start": "node index.js"
},

Now save and close package.json and run

npm start

to start your application.

You want to run a command like file my.pdf using NodeJS child-process.exec and get its stdout after it’s finished.

Solution:

TL;DR: (await exec('file my.pdf')).stdout

We’re using child-process-promise here in order to simplify our implementation. Install it using npm i --save child-process-promise !

const { exec } = require('child-process-promise');

async function run () {
    const ret = await exec(`file my.pdf`);
    return ret.stdout;
}

run().then(console.log).catch(console.error);

You can also use .stderr instead of .stdout to get the stderr output as a string

Problem:

You are trying to build your webpack project, but you see an error message like this:

/home/uli/project/node_modules/webpack-cli/bin/config-yargs.js:89
                                describe: optionsSchema.definitions.output.properties.path.description,
                                                                           ^

TypeError: Cannot read property 'properties' of undefined
    at module.exports (/home/uli/project/node_modules/webpack-cli/bin/config-yargs.js:89:48)
    at /home/uli/project/node_modules/webpack-cli/bin/webpack.js:60:27
    at Object.<anonymous> (/home/uli/project/node_modules/webpack-cli/bin/webpack.js:515:3)
    at Module._compile (internal/modules/cjs/loader.js:723:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:734:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)
    at Module.require (internal/modules/cjs/loader.js:659:17)
    at require (internal/modules/cjs/helpers.js:22:18)

Solution:

This is a known bug in webpack 4.20.0 – you can circumvent this issue by using webpack 4.19.0.

Look for a line like

"webpack": "^4.7.0",

in your package.json. The caret (^) allows npm to use any 4.x.x version – including the broken 4.20.0.

Replace the aforementioned line by

"webpack": "4.19.0",

to use only webpack 4.19.0.

After that, run npm install and retry building your application.

This snippet reads a XLSX file using js-xlsx, sets the C2 cell to abc123 and writes the result to another file:

const XLSX = require('xlsx');

const table = XLSX.readFile('mytable.xlsx');
// Use first sheet
const sheet = table.Sheets[table.SheetNames[0]];

// Option 1: If you have numeric row and column indexes
sheet[XLSX.utils.encode_cell({r: 1 /* 2 */, c: 2 /* C */})] = {t: 's' /* type: string */, v: 'abc123' /* value */};
// Option 2: If you have a cell coordinate like 'C2' or 'D15'
sheet['C2'] = {t: 's' /* type: string */, v: 'abc123' /* value */};

XLSX.writeFile(table, 'result.xlsx');

This snippet allows you to easily iterate over rows in any XLSX files using the js-xlsx library (in this example we don’t iterate over all columns but rather only get the B column as an example):

const table = XLSX.readFile('mytable.xlsx');
const sheet = table.Sheets[table.SheetNames[0]];

var range = XLSX.utils.decode_range(sheet['!ref']);
for (let rowNum = range.s.r; rowNum <= range.e.r; rowNum++) {
    // Example: Get second cell in each row, i.e. Column "B"
    const secondCell = sheet[XLSX.utils.encode_cell({r: rowNum, c: 1})];
    // NOTE: secondCell is undefined if it does not exist (i.e. if its empty)
    console.log(secondCell); // secondCell.v contains the value, i.e. string or number
}