Power Automate Expression Builder

What Power Automate expressions actually are

Expressions in Power Automate are not a bespoke formula language. They are the Workflow Definition Language, the same engine Azure Logic Apps runs on, which is why Logic Apps documentation answers Power Automate questions and why the function list is identical in both. Everything you type into the expression box is a function call over JSON: there are no statements, no assignment, and no loops. If you find yourself wanting those, the answer is an action, not a longer expression.

The most common failure: [ versus ?[

triggerBody()['Email'] and triggerBody()?['Email']behave identically right up until the property is missing, at which point the first throws and fails the run and the second returns null. An empty SharePoint column, an optional form field nobody filled in, a lookup that was cleared — each produces a property that is absent rather than empty. Flows built and tested against complete data fail weeks later on the first incomplete record. Use the null-safe ?[] accessor unless you specifically want a hard failure, and wrap the result in coalesce() when you need a default.

Action names are not what the designer shows you

References such as body('Get_items') and items('Apply_to_each')use the action's internal name, where every space becomes an underscore. The designer displays “Apply to each”; the expression needs Apply_to_each. Rename an action after writing expressions against it and every reference breaks — the flow saves without complaint and fails at run time with an unknown-action error. Settle on action names before you write expressions against them.

Time zones take Windows IDs, not IANA ones

convertFromUtc() and convertTimeZone()expect Windows time zone identifiers — Arabian Standard Time, India Standard Time, GMT Standard Time — not the IANA names (Asia/Qatar, Europe/London) that most other modern platforms use. An IANA string fails at run time rather than falling back to something sensible. The format string is ICU, and note thatMM is month while mm is minutes, which is the other dependable source of date bugs here.

Type coercion is stricter than it looks

int('') throws rather than returning zero, and so does int(null). A guarded conversion — if(empty(x), 0, int(x))— is the idiom worth learning early. Comparisons are typed too: equals(1, '1') is false, which matters when a value arrives as a string from an HTTP response and is compared against a number from a SharePoint column.

Working with collections without a loop

There is no filter() expression function. Filtering an array is the Filter array action and reshaping one is the Select action. The expression functions that do operate on collections are first(), last(), take(), skip(), union(), intersection(), join() and contains(). Reaching for an Apply to each purely to find one matching item is the most common performance mistake in Power Automate: a Filter array followed by first() does the same work in one action instead of one iteration per record.

How to debug one

Build the expression in a Compose action first. Compose does nothing except evaluate its input and show the result in the run history, so you can see what a subexpression returns before nesting it three levels deep. When a run fails, open the failed action and read its Inputsrather than the error text — the error tells you a property was not found, the inputs tell you what the shape actually was.