Lodash partial Argument Execution Priority

When working with functional composition in Lodash, the _.partial method creates a new function with predefined arguments. In terms of execution priority, partially applied arguments take precedence by occupying the initial positions of the target function's parameter list, while runtime arguments are appended afterward. However, this left-to-right priority order can be altered using placeholders, which allow runtime arguments to fill specific slots before unassigned arguments are appended to the end.

Default Argument Precedence

Under normal operation, _.partial establishes a fixed left-to-right argument order. The arguments supplied during the definition of _.partial are placed at the beginning of the parameter list (index 0, 1, 2, and so on).

When the resulting partially applied function is invoked, any arguments passed at runtime are appended after the partially applied ones:

const _ = require('lodash');

function showOrder(a, b, c) {
  console.log(`a: ${a}, b: ${b}, c: ${c}`);
}

// Partially apply 'first' to parameter 'a'
const partialFn = _.partial(showOrder, 'first');

// Runtime arguments 'second' and 'third' fill 'b' and 'c'
partialFn('second', 'third');
// Output: a: first, b: second, c: third

In this model, the partially applied arguments hold higher positional priority over runtime arguments.

Altering Priority with Placeholders

Lodash allows you to override default positional precedence by using placeholders (_ or _.partial.placeholder). When a placeholder is present in the partial application call, Lodash alters the resolution priority:

1. Placeholder Resolution: Runtime arguments are first consumed left-to-right to replace any placeholders defined in _.partial.
2. Trailing Arguments: Once all placeholders are filled, any remaining runtime arguments are appended to the end of the argument list.

// Reserve the first parameter with a placeholder
const partialWithHole = _.partial(showOrder, _, 'second');

// 'first' fills the placeholder; 'third' is appended to the end
partialWithHole('first', 'third');
// Output: a: first, b: second, c: third

Contrast with _.partialRight

If your implementation requires runtime arguments to take positional priority from the beginning of the parameter list, Lodash provides _.partialRight. With _.partialRight, the partially applied arguments are appended to the tail of the parameter list, allowing runtime arguments to populate the initial parameters first:

const partialRightFn = _.partialRight(showOrder, 'third');

partialRightFn('first', 'second');
// Output: a: first, b: second, c: third

Summary of Priority Rules

• Fixed Partial Arguments: Bound to specific positions from left to right at initialization.
• Placeholders: Receive the highest priority for incoming runtime arguments until all placeholders are satisfied.
• Runtime Arguments: First fill placeholders in sequential order, then populate any remaining parameters at the end of the argument list.