One liner to flatten nested object

Question

Here you go:

Object.assign({}, ...function _flatten(o) { return [].concat(...Object.keys(o).map(k => typeof o[k] === 'object' ? _flatten(o[k]) : ({[k]: o[k]})))}(yourObject))

Summary: recursively create an array of one-property objects, then combine them all with Object.assign.

This uses ES6 features including Object.assign or the spread operator, but it should be easy enough to rewrite not to require them.

For those who don't care about the one-line craziness and would prefer to be able to actually read it (depending on your definition of readability):

Object.assign(
  {}, 
  ...function _flatten(o) { 
    return [].concat(...Object.keys(o)
      .map(k => 
        typeof o[k] === 'object' ?
          _flatten(o[k]) : 
          ({[k]: o[k]})
      )
    );
  }(yourObject)
)

Simplified readable example, no dependencies

/**
 * Flatten a multidimensional object
 *
 * For example:
 *   flattenObject({ a: 1, b: { c: 2 } })
 * Returns:
 *   { a: 1, c: 2}
 */
export const flattenObject = (obj) => {
  const flattened = {}

  Object.keys(obj).forEach((key) => {
    if (typeof obj[key] === 'object' && obj[key] !== null) {
      Object.assign(flattened, flattenObject(obj[key]))
    } else {
      flattened[key] = obj[key]
    }
  })

  return flattened
}

Working example: https://jsfiddle.net/webbertakken/jn613d8p/2/

Here is a true, crazy one-liner that flats the nested object recursively:

const flatten = (obj, roots=[], sep='.') => Object.keys(obj).reduce((memo, prop) => Object.assign({}, memo, Object.prototype.toString.call(obj[prop]) === '[object Object]' ? flatten(obj[prop], roots.concat([prop]), sep) : {[roots.concat([prop]).join(sep)]: obj[prop]}), {})

Multiline version, explained:

// $roots keeps previous parent properties as they will be added as a prefix for each prop.
// $sep is just a preference if you want to seperate nested paths other than dot.
const flatten = (obj, roots = [], sep = '.') => Object
  // find props of given object
  .keys(obj)
  // return an object by iterating props
  .reduce((memo, prop) => Object.assign(
    // create a new object
    {},
    // include previously returned object
    memo,
    Object.prototype.toString.call(obj[prop]) === '[object Object]'
      // keep working if value is an object
      ? flatten(obj[prop], roots.concat([prop]), sep)
      // include current prop and value and prefix prop with the roots
      : {[roots.concat([prop]).join(sep)]: obj[prop]}
  ), {})

An example:

const obj = {a: 1, b: 'b', d: {dd: 'Y'}, e: {f: {g: 'g'}}}
const flat = flatten(obj)
{
  'a': 1, 
  'b': 'b', 
  'd.dd': 'Y', 
  'e.f.g': 'g'
}

Happy one-liner day!

It's not quite a one liner, but here's a solution that doesn't require anything from ES6. It uses underscore's extend method, which could be swapped out for jQuery's.

function flatten(obj) {
    var flattenedObj = {};
    Object.keys(obj).forEach(function(key){
        if (typeof obj[key] === 'object') {
            $.extend(flattenedObj, flatten(obj[key]));
        } else {
            flattenedObj[key] = obj[key];
        }
    });
    return flattenedObj;    
}

Here are vanilla solutions that work for arrays, primitives, regular expressions, functions, any number of nested object levels, and just about everything else I could throw at them. The first overwrites property values in the manner that you would expect from Object.assign.

((o) => {
  return o !== Object(o) || Array.isArray(o) ? {}
    : Object.assign({}, ...function leaves(o) {
    return [].concat.apply([], Object.entries(o)
      .map(([k, v]) => {
        return (( !v || typeof v !== 'object'
            || !Object.keys(v).some(key => v.hasOwnProperty(key))
            || Array.isArray(v))
          ? {[k]: v}
          : leaves(v)
        );
      })
    );
  }(o))
})(o)

The second accumulates values into an array.

((o) => {
  return o !== Object(o) || Array.isArray(o) ? {}
    : (function () {
      return Object.values((function leaves(o) {
        return [].concat.apply([], !o ? [] : Object.entries(o)
          .map(([k, v]) => {
            return (( !v || typeof v !== 'object'
                || !Object.keys(v).some(k => v.hasOwnProperty(k))
                || (Array.isArray(v) && !v.some(el => typeof el === 'object')))
              ? {[k]: v}
              : leaves(v)
            );
          })
        );
      }(o))).reduce((acc, cur) => {
        return ((key) => {
          acc[key] = !acc[key] ? [cur[key]]
            : new Array(...new Set(acc[key].concat([cur[key]])))
        })(Object.keys(cur)[0]) ? acc : acc
      }, {})
    })(o);
})(o)

Also please do not include code like this in production as it is terribly difficult to debug.

function leaves1(o) {
  return ((o) => {
    return o !== Object(o) || Array.isArray(o) ? {}
      : Object.assign({}, ...function leaves(o) {
      return [].concat.apply([], Object.entries(o)
        .map(([k, v]) => {
          return (( !v || typeof v !== 'object'
              || !Object.keys(v).some(key => v.hasOwnProperty(key))
              || Array.isArray(v))
            ? {[k]: v}
            : leaves(v)
          );
        })
      );
    }(o))
  })(o);
}

function leaves2(o) {
  return ((o) => {
    return o !== Object(o) || Array.isArray(o) ? {}
      : (function () {
        return Object.values((function leaves(o) {
          return [].concat.apply([], !o ? [] : Object.entries(o)
            .map(([k, v]) => {
              return (( !v || typeof v !== 'object'
                  || !Object.keys(v).some(k => v.hasOwnProperty(k))
                  || (Array.isArray(v) && !v.some(el => typeof el === 'object')))
                ? {[k]: v}
                : leaves(v)
              );
            })
          );
        }(o))).reduce((acc, cur) => {
          return ((key) => {
            acc[key] = !acc[key] ? [cur[key]]
              : new Array(...new Set(acc[key].concat([cur[key]])))
          })(Object.keys(cur)[0]) ? acc : acc
        }, {})
      })(o);
  })(o);
}

const obj = {
  l1k0: 'foo',
  l1k1: {
    l2k0: 'bar',
    l2k1: {
      l3k0: {},
      l3k1: null
    },
    l2k2: undefined
  },
  l1k2: 0,
  l2k3: {
    l3k2: true,
    l3k3: {
      l4k0: [1,2,3],
      l4k1: [4,5,'six', {7: 'eight'}],
      l4k2: {
        null: 'test',
        [{}]: 'obj',
        [Array.prototype.map]: Array.prototype.map,
        l5k3: ((o) => (typeof o === 'object'))(this.obj),
      }
    }
  },
  l1k4: '',
  l1k5: new RegExp(/[\s\t]+/g),
  l1k6: function(o) { return o.reduce((a,b) => a+b)},
  false: [],
}
const objs = [null, undefined, {}, [], ['non', 'empty'], 42, /[\s\t]+/g, obj];

objs.forEach(o => {
  console.log(leaves1(o));
});
objs.forEach(o => {
  console.log(leaves2(o));
});

I like this code because it's a bit easier to understand.

Edit: I added some functionality I needed, so now it's a bit harder to understand.

const data = {
  a: "a",
  b: {
    c: "c",
    d: {
      e: "e",
      f: [
        "g",
        {
          i: "i",
          j: {},
          k: []
        }
      ]
    }
  }
};

function flatten(data, response = {}, flatKey = "", onlyLastKey = false) {
  for (const [key, value] of Object.entries(data)) {
    let newFlatKey;
    if (!isNaN(parseInt(key)) && flatKey.includes("[]")) {
      newFlatKey = (flatKey.charAt(flatKey.length - 1) == "." ? flatKey.slice(0, -1) : flatKey) + `[${key}]`;
    } else if (!flatKey.includes(".") && flatKey.length > 0) {
      newFlatKey = `${flatKey}.${key}`;
    } else {
      newFlatKey = `${flatKey}${key}`;
    }
    if (typeof value === "object" && value !== null && Object.keys(value).length > 0) {
      flatten(value, response, `${newFlatKey}.`, onlyLastKey);
    } else {
      if(onlyLastKey){
        newFlatKey = newFlatKey.split(".").pop();
      }
      if (Array.isArray(response)) {
        response.push({
          [newFlatKey.replace("[]", "")]: value
        });
      } else {
        response[newFlatKey.replace("[]", "")] = value;
      }
    }
  }
  return response;
}

console.log(flatten(data));
console.log(flatten(data, {}, "data"));
console.log(flatten(data, {}, "data[]"));
console.log(flatten(data, {}, "data", true));
console.log(flatten(data, {}, "data[]", true));
console.log(flatten(data, []));
console.log(flatten(data, [], "data"));
console.log(flatten(data, [], "data[]"));
console.log(flatten(data, [], "data", true));
console.log(flatten(data, [], "data[]", true));

Demo https://stackblitz.com/edit/typescript-flatter

For insinde a typescript class use:

function flatten(data: any, response = {}, flatKey = "", onlyLastKey = false) {
  for (const [key, value] of Object.entries(data)) {
    let newFlatKey: string;
    if (!isNaN(parseInt(key)) && flatKey.includes("[]")) {
      newFlatKey = (flatKey.charAt(flatKey.length - 1) == "." ? flatKey.slice(0, -1) : flatKey) + `[${key}]`;
    } else if (!flatKey.includes(".") && flatKey.length > 0) {
      newFlatKey = `${flatKey}.${key}`;
    } else {
      newFlatKey = `${flatKey}${key}`;
    }
    if (typeof value === "object" && value !== null && Object.keys(value).length > 0) {
      flatten(value, response, `${newFlatKey}.`, onlyLastKey);
    } else {
      if(onlyLastKey){
        newFlatKey = newFlatKey.split(".").pop();
      }
      if (Array.isArray(response)) {
        response.push({
          [newFlatKey.replace("[]", "")]: value
        });
      } else {
        response[newFlatKey.replace("[]", "")] = value;
      }
    }
  }
  return response;
}