Response Templating - Conditional Logic and Iteration

Taking actions conditionally and looping over collections of data are very common requirements from a templating system. This article explains how these are achieved in MockLab.

Conditional logic with if / else and unless

Handlebars provides a set of core helpers that implement if / else if / else logic of the kind found in many programming languages.

As with most implementations of if, the simples form is to take an action only if the condition is true:

{{#if showDetails}}
  <div id="details">...</div>

An else clause can be used:

{{#if showDetails}}
  <div id="details">...</div>
  <div id="details" class="hidden">...</div>

And any number of else if clauses can also be added:

{{#if showVariantA}}
  <div id="var-a">...</div>
{{else if showVariantB}}
  <div id="var-b">...</div>
{{else if showVariantC}}
  <div id="var-c">...</div>
  <div id="default-var">...</div>

Finally, you can take an action if a condition is false using unless:

{{#unless hideDetails}}
  <div id="details">...</div>

Comparison helpers

The if, else if and unless helpers all take a single boolean value as their parameter. In practice you often need to derive that value by comparing other values, and for this we have a set of helpers implementing common comparison operations.

For instance if you needed to check that a variable equalled a particular string you would use the eq helper:

{{#eq name 'Dan'}}
  <div id="dan">...</div>

You can nest comparison helpers inside the if helper:

{{#if (eq name 'Dan')}}
  <div id="dan">...</div>

You can also use comparison helpers with else:

{{#eq name 'Dan'}}
  <div id="dan">...</div>
{{else eq name 'Mark'}}
  <div id="mark">...</div>
  <div id="anon">...</div>

The following comparison helpers are available:

eq - equal

{{#eq name 'Jeff'}}...{{/eq}}

neq - not equal

{{#neq name 'Jeff'}}...{{/neq}}

gt - greater than

{{#gt itemCount 3}}...{{/gt}}

gte - greater than or equal to

{{#gte itemCount 3}}...{{/gte}}

lt - less than

{{#lt itemCount 3}}...{{/lt}}

lte - less than or equal

{{#lte itemCount 3}}...{{/lte}}

and - logical AND

{{#and (lt itemCount 10) (gt itemCount 5)}}...{{/and}}

or - logical OR

{{#or (eq itemCount 1) (eq itemCount 2)}}...{{/or}}

not - logical NOT

{{#not (eq itemCount 1)}}...{{/not}}


You can loop over collections of data using the each helper.

{{#each request.query.things as |thing|}}
  thing: {{{thing}}}

Iterating over JSON and XML elements

The jsonPath and xPath helpers both output collections so these can be used in an each loop. See Working with JSON and Working with XML for details.

Detecting the first and last element while looping

Often it can be useful to know when you’re processing the first or last element in a collection e.g. so that you can decide whether to output a separate character.

You can do this using the @first and @last variables that are automatically provided to the scope inside the each block.

For instance, if you wanted to output a list of JSON objects, separated with commas and avoiding an extraneous comma at the end:

{{#each (jsonPath request.body '$.things') as |thing|}}
  {{#if @last}}
    { "thing": {{{thing}}} }
    { "thing": {{{thing}}} },

Getting the loop index

The each helper also creates an @index variable in its scope which you can use to get at the (zero-indexed) element counter:

{{#each (jsonPath request.body '$.things') as |thing|}}
  {{@index}}: {{thing}}