Accessing a JavaScript's object property without knowing that property name

Use for...in

You can also use the for in loop:

data.forEach( function ( m ) {
  for ( const key in m ) {
    if (m.hasOwnProperty(key)) { // Only print properties that have not been inherited
      console.log( key );        // "who"
      console.log( m[key] );     // "Arthur"
    }
  }

});

The above would also work for multiple key: value pairs in your object i.e:

[ {"who": "Arthur", "who": "Fred"}, {"who": "Craig"}, ]

Issues with the above:

• Order is not guaranteed when iterating over objects
• If not using hasOwnProperty you might iterate over inherited properties

Use for...of

Another good option of iterating over objects is with for..of and Object.enteries()

for ( const [key, value] of Object.entries(data) ) {
  console.log( key ); // "who"
  console.log( value ); // "Arthur"  
}

• Only iterates over enumerable props

If you always expect these objects to have only one property, you could do something like this:

var name, person;
for (person in data) {
    for (name in data[person]) {
        console.log(data[person][name]);
    }
}

This would enumerate through each property of each person in the data. Because there is only one property per person (I assume), it will just enumerate that one property and stop, allowing you to use that property regardless of its name.