function validator(operation, index, document, existingPathFragment) {
    if (typeof operation !== 'object' || operation === null || Array.isArray(operation)) {
        throw new exports.JsonPatchError('Operation is not an object', 'OPERATION_NOT_AN_OBJECT', index, operation, document);
    }
    else if (!objOps[operation.op]) {
        throw new exports.JsonPatchError('Operation `op` property is not one of operations defined in RFC-6902', 'OPERATION_OP_INVALID', index, operation, document);
    }
    else if (typeof operation.path !== 'string') {
        throw new exports.JsonPatchError('Operation `path` property is not a string', 'OPERATION_PATH_INVALID', index, operation, document);
    }
    else if (operation.path.indexOf('/') !== 0 && operation.path.length > 0) {
        // paths that aren't empty string should start with "/"
        throw new exports.JsonPatchError('Operation `path` property must start with "/"', 'OPERATION_PATH_INVALID', index, operation, document);
    }
    else if ((operation.op === 'move' || operation.op === 'copy') && typeof operation.from !== 'string') {
        throw new exports.JsonPatchError('Operation `from` property is not present (applicable in `move` and `copy` operations)', 'OPERATION_FROM_REQUIRED', index, operation, document);
    }
    else if ((operation.op === 'add' || operation.op === 'replace' || operation.op === 'test') && operation.value === undefined) {
        throw new exports.JsonPatchError('Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)', 'OPERATION_VALUE_REQUIRED', index, operation, document);
    }
    else if ((operation.op === 'add' || operation.op === 'replace' || operation.op === 'test') && helpers_js_1.hasUndefined(operation.value)) {
        throw new exports.JsonPatchError('Operation `value` property is not present (applicable in `add`, `replace` and `test` operations)', 'OPERATION_VALUE_CANNOT_CONTAIN_UNDEFINED', index, operation, document);
    }
    else if (document) {
        if (operation.op == "add") {
            var pathLen = operation.path.split("/").length;
            var existingPathLen = existingPathFragment.split("/").length;
            if (pathLen !== existingPathLen + 1 && pathLen !== existingPathLen) {
                throw new exports.JsonPatchError('Cannot perform an `add` operation at the desired path', 'OPERATION_PATH_CANNOT_ADD', index, operation, document);
            }
        }
        else if (operation.op === 'replace' || operation.op === 'remove' || operation.op === '_get') {
            if (operation.path !== existingPathFragment) {
                throw new exports.JsonPatchError('Cannot perform the operation at a path that does not exist', 'OPERATION_PATH_UNRESOLVABLE', index, operation, document);
            }
        }
        else if (operation.op === 'move' || operation.op === 'copy') {
            var existingValue = { op: "_get", path: operation.from, value: undefined };
            var error = validate([existingValue], document);
            if (error && error.name === 'OPERATION_PATH_UNRESOLVABLE') {
                throw new exports.JsonPatchError('Cannot perform the operation from a path that does not exist', 'OPERATION_FROM_UNRESOLVABLE', index, operation, document);
            }
        }
    }
}
Validates a single operation. Called from `jsonpatch.validate`. Throws `JsonPatchError` in case of an error.
@param {object} operation - operation object (patch)
@param {number} index - index of operation in the sequence
@param {object} [document] - object where the operation is supposed to be applied
@param {string} [existingPathFragment] - comes along with `document`